View source for Transmission of COVID-19

{{Short description|Mechanisms that spread coronavirus disease 2019}}
{{Use dmy dates|date=April 2023}}
{{cs1 config |name-list-style=vanc |display-authors=6}}
{{Update|date=July 2024}}
<noinclude><!-- LEAD PRAGRAPHS 1-4 ARE TRANSCLUDED INTO COVID-19 AND COVID-19 Pandemic -->
{{Infobox medical condition
| name          = Transmission of COVID-19
| synonyms      = Mode of spread of COVID-19
| image         = Covid-19_Aerosol.jpg
| image_size    = 240px
| caption       = {{longitem|The respiratory route of spread of COVID-19, encompassing larger droplets and aerosols}}
| specialty     = [[Infection prevention and control]]
| types         = [[Respiratory droplet]], [[airborne transmission]], [[fomites]]
| prevention    = Face coverings, [[quarantine]], physical/[[social distancing]], ventilation, disinfection, hand washing, [[COVID-19 vaccine|vaccination]]
}}
The '''transmission of COVID-19''' is the [[Transmission (medicine)|passing]] of [[coronavirus disease 2019]] from person to person. </noinclude>COVID-19 is mainly transmitted when people breathe in air contaminated by [[Drop (liquid)|droplets]]/[[aerosol]]s and small [[Airborne transmission|airborne]] particles containing the virus. Infected people exhale those particles as they breathe, talk, cough, sneeze, or sing.<ref name="Wang_2021">{{cite journal | vauthors = Wang CC, Prather KA, Sznitman J, Jimenez JL, Lakdawala SS, Tufekci Z, Marr LC | title = Airborne transmission of respiratory viruses | language = EN | journal = Science | volume = 373 | issue = 6558 | date = August 2021 | pmid = 34446582 | doi = 10.1126/science.abd9149 | pmc = 8721651 | doi-access = free }}</ref><ref name="Greenhalgh_2021">{{cite journal | vauthors = Greenhalgh T, Jimenez JL, Prather KA, Tufekci Z, Fisman D, Schooley R | title = Ten scientific reasons in support of airborne transmission of SARS-CoV-2 | journal = Lancet | volume = 397 | issue = 10285 | pages = 1603–1605 | date = May 2021 | pmid = 33865497 | pmc = 8049599 | doi = 10.1016/s0140-6736(21)00869-2 }}</ref><ref name="AR2021">{{cite journal | vauthors = Bourouiba L |title=Fluid Dynamics of Respiratory Infectious Diseases |journal=Annual Review of Biomedical Engineering |date=13 July 2021 |volume=23 |issue=1 |pages=547–577 |doi=10.1146/annurev-bioeng-111820-025044 |pmid=34255991 |url=https://www.annualreviews.org/doi/10.1146/annurev-bioeng-111820-025044 |access-date=7 September 2021|hdl=1721.1/131115 |s2cid=235823756 |hdl-access=free }}</ref><ref>{{cite journal |last1=Stadnytskyi |first1=Valentyn |last2=Bax |first2=Christina E. |last3=Bax |first3=Adriaan |last4=Anfinrud |first4=Philip |title=The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission |journal=Proceedings of the National Academy of Sciences |date=2 June 2020 |volume=117 |issue=22 |pages=11875–11877 |doi=10.1073/pnas.2006874117 |pmid=32404416 |pmc=7275719 |bibcode=2020PNAS..11711875S |doi-access=free }}</ref> Transmission is more likely the closer people are. However, infection can occur over longer distances, particularly indoors.<ref name="Wang_2021" /><ref name="Miller_2021">{{cite journal | vauthors = Miller SL, Nazaroff WW, Jimenez JL, Boerstra A, Buonanno G, Dancer SJ, Kurnitski J, Marr LC, Morawska L, Noakes C | title = Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event | journal = Indoor Air | volume = 31 | issue = 2 | pages = 314–323 | date = March 2021 | pmid = 32979298 | pmc = 7537089 | doi = 10.1111/ina.12751 | bibcode = 2021InAir..31..314M }}</ref>

The transmission of the virus is carried out through virus-laden fluid particles, or droplets, which are created in the respiratory tract, and they are expelled by the mouth and the nose. There are three types of transmission: “droplet” and “contact”, which are associated with large droplets, and “airborne”, which is associated with small droplets.<ref name=":1">{{cite journal |last=Mittal |first=Rajat |date=2020 |title=The flow physics of COVID-19 |url=|journal=Journal of Fluid Mechanics |volume=894|doi=10.1017/jfm.2020.330 |arxiv=2004.09354 |bibcode=2020JFM...894F...2M |s2cid=215827809 }}</ref> If the droplets are above a certain critical size, they settle faster than they [[evaporate]], and therefore they [[contaminate]] surfaces surrounding them.<ref name=":1" /> Droplets that are below a certain critical size, generally thought to be <100μm diameter, evaporate faster than they settle; due to that fact, they form respiratory aerosol particles that remain airborne for a long period of time over extensive distances. SARS-CoV-2 is the virus responsible for COVID-19. spreading in 2024. It belongs to the coronavirus family, which also includes viruses that cause diseases such as the common cold, Middle East Respiratory Syndrome (MERS), and Severe Acute Respiratory Syndrome (SARS).<ref>{{Cite web |last=Raja |first=Nomi |date=2024-08-08 |title=Covid-19 symptoms in 2024 |url=https://wavepagex.com/covid-19-symptoms-in-2024/ |url-status=live |website=wavepagex.com}}</ref>.<ref name=":1" /><ref name="Wang_2021" />

[[Infectivity]] can begin four to five days before the onset of symptoms.<ref name=HeCorrection>{{cite journal |last1=He |first1=Xi |last2=Lau |first2=Eric H. Y. |last3=Wu |first3=Peng |last4=Deng |first4=Xilong |last5=Wang |first5=Jian |last6=Hao |first6=Xinxin |last7=Lau |first7=Yiu Chung |last8=Wong |first8=Jessica Y. |last9=Guan |first9=Yujuan |last10=Tan |first10=Xinghua |last11=Mo |first11=Xiaoneng |last12=Chen |first12=Yanqing |last13=Liao |first13=Baolin |last14=Chen |first14=Weilie |last15=Hu |first15=Fengyu |last16=Zhang |first16=Qing |last17=Zhong |first17=Mingqiu |last18=Wu |first18=Yanrong |last19=Zhao |first19=Lingzhai |last20=Zhang |first20=Fuchun |last21=Cowling |first21=Benjamin J. |last22=Li |first22=Fang |last23=Leung |first23=Gabriel M. |title=Author Correction: Temporal dynamics in viral shedding and transmissibility of COVID-19 |journal=Nature Medicine |date=September 2020 |volume=26 |issue=9 |pages=1491–1493 |doi=10.1038/s41591-020-1016-z|pmid=32770170 |pmc=7413015 |s2cid=221050261 }}</ref> Infected people can spread the disease even if they are pre-symptomatic or [[asymptomatic]].<ref name="CDNASoNG">{{cite web |author1=Communicable Diseases Network Australia |title=Coronavirus Disease 2019 (COVID-19): CDNA National Guidelines for Public Health Units |url=https://www1.health.gov.au/internet/main/publishing.nsf/Content/cdna-song-novel-coronavirus.htm |publisher=Communicable Diseases Network Australia/Australian Government Department of Health |version=5.1}}</ref>  Most commonly, the peak viral load in [[upper respiratory tract]] samples occurs close to the time of symptom onset and declines after the first week after symptoms begin.<ref name="CDNASoNG"/> Current evidence suggests a duration of viral shedding and the period of infectiousness of up to ten days following symptom onset for people with mild to moderate COVID-19, and up to 20 days for persons with severe COVID-19, including immunocompromised people.<ref name=CDCClinicalQuestions>{{cite web | url = https://www.cdc.gov/coronavirus/2019-ncov/hcp/faq.html | title = Clinical Questions about COVID-19: Questions and Answers | publisher = [[Centers for Disease Control and Prevention]] | date =4 March 2021}}</ref><ref name="CDNASoNG" />

Infectious particles range in size from [[aerosol]]s that remain suspended in the air for long periods of time to larger [[Respiratory droplet|droplets]] that remain airborne briefly or fall to the ground.<ref name="CDCScientificBriefTransmission"/><ref name="who_transmitted"/><ref name="WHO-ECDC-CDC-UK-Ca-Aus-Annals">{{•}}{{cite web|title=COVID-19: epidemiology, virology and clinical features |url=https://www.gov.uk/government/publications/wuhan-novel-coronavirus-background-information/wuhan-novel-coronavirus-epidemiology-virology-and-clinical-features|access-date=18 October 2020|website=GOV.UK}}<br />{{•}}{{cite web|author1=Communicable Diseases Network Australia|title=Coronavirus Disease 2019 (COVID-19) - CDNA Guidelines for Public Health Units|url=https://www1.health.gov.au/internet/main/publishing.nsf/Content/cdna-song-novel-coronavirus.htm|access-date=17 May 2021|series=Version 4.4|publisher=Australian Government Department of Health}}<br />{{•}}{{cite web|author=Public Health Agency of Canada|date=3 November 2020|title=COVID-19: Main modes of transmission|url=https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/health-professionals/main-modes-transmission.html|access-date=18 May 2021|website=aem}}<br />{{•}}{{cite web |title=Transmission of COVID-19|url=https://www.ecdc.europa.eu/en/covid-19/latest-evidence/transmission|access-date=18 May 2021|website=European Centre for Disease Prevention and Control|date=26 January 2021 }}<br />{{•}}{{cite journal |vauthors=Meyerowitz EA, Richterman A, Gandhi RT, Sax PE|date=January 2021|title=Transmission of SARS-CoV-2: A Review of Viral, Host, and Environmental Factors|journal=Annals of Internal Medicine|volume=174|issue=1|pages=69–79|doi=10.7326/M20-5008 |issn=0003-4819|pmc=7505025|pmid=32941052}}</ref><ref name="Tang_2021" /> Additionally, COVID-19 research has redefined the traditional understanding of how respiratory viruses are transmitted.<ref name="Tang_2021">{{cite journal | vauthors = Tang JW, Marr LC, Li Y, Dancer SJ | title = Covid-19 has redefined airborne transmission | journal = BMJ | volume = 373 | pages = n913 | date = April 2021 | pmid = 33853842 | doi = 10.1136/bmj.n913 | doi-access = free }}</ref><ref name="Morawska_2021">{{cite journal | vauthors = Morawska L, Allen J, Bahnfleth W, Bluyssen PM, Boerstra A, Buonanno G, Cao J, Dancer SJ, Floto A, Franchimon F, Greenhalgh T, Haworth C, Hogeling J, Isaxon C, Jimenez JL, Kurnitski J, Li Y, Loomans M, Marks G, Marr LC, Mazzarella L, Melikov AK, Miller S, Milton DK, Nazaroff W, Nielsen PV, Noakes C, Peccia J, Prather K, Querol X, Sekhar C, Seppänen O, Tanabe SI, Tang JW, Tellier R, Tham KW, Wargocki P, Wierzbicka A, Yao M | title = A paradigm shift to combat indoor respiratory infection | journal = Science | volume = 372 | issue = 6543 | pages = 689–691 | date = May 2021 | pmid = 33986171 | doi = 10.1126/science.abg2025 | bibcode = 2021Sci...372..689M | s2cid = 234487289 | url = https://eprints.qut.edu.au/210390/1/83856124.pdf | access-date = 14 June 2021 | archive-date = 6 December 2021 | archive-url = https://web.archive.org/web/20211206101205/https://eprints.qut.edu.au/210390/1/83856124.pdf | url-status = dead }}</ref> The largest droplets of respiratory fluid do not travel far, but can be inhaled or land on mucous membranes on the eyes, nose, or mouth to infect.<ref name="WHO-ECDC-CDC-UK-Ca-Aus-Annals" /> Aerosols are highest in concentration when people are in close proximity, which leads to easier viral transmission when people are physically close,<ref name="WHO-ECDC-CDC-UK-Ca-Aus-Annals" /><ref name="Tang_2021" /><ref name="Morawska_2021" /> but [[airborne transmission]] can occur at longer distances, mainly in locations that are poorly ventilated;<ref name="WHO-ECDC-CDC-UK-Ca-Aus-Annals" /> in those conditions small particles can remain suspended in the air for minutes to hours.<ref name="WHO-ECDC-CDC-UK-Ca-Aus-Annals" /><ref>{{cite journal | author1 = Biswas Riddhideep | author2 = Pal Anish | author3 = Pal Ritam | author4 = Sarkar Sourav | author5 = Mukhopadhyay Achintya| title = Risk assessment of COVID infection by respiratory droplets from cough for various ventilation scenarios inside an elevator: An OpenFOAM-based computational fluid dynamics analysis | journal = Physics of Fluids |year = 2022 | volume = 34 |issue = 1 | pages = 013318 | doi = 10.1063/5.0073694 |pmid = 35340680 | pmc = 8939552 |arxiv = 2109.12841 |bibcode = 2022PhFl...34a3318B |s2cid = 245828044 | url=}}</ref>

The number of people generally infected by one infected person varies,<ref name="AnnalsTransmit2">{{cite journal | vauthors = Meyerowitz EA, Richterman A, Gandhi RT, Sax PE | title = Transmission of SARS-CoV-2: A Review of Viral, Host, and Environmental Factors | journal = Annals of Internal Medicine | volume = 174 | issue = 1 | pages = 69–79 | date = January 2021 | pmid = 32941052 | pmc = 7505025 | doi = 10.7326/M20-5008 | issn=0003-4819 }}</ref> <!-- as only 10 to 20% of people are responsible for the disease's spread,<ref name="Lessler_2021">{{cite web |title=Overdispersion of COVID-19 |url=https://www.jhsph.edu/covid-19/articles/overdispersion-of-covid-19.html |access-date=11 May 2021 |website=Johns Hopkins Bloomberg School of Public Health |vauthors=Lessler J, Grantz K}}</ref> --> but it is estimated that the [[basic reproduction number|R<sub>0</sub>]] ("R nought" or "R zero") number is around 2.5.<ref>{{cite web |last=CDC |date=11 February 2020 |title=Healthcare Workers |url=https://www.cdc.gov/coronavirus/2019-ncov/hcp/planning-scenarios.html |access-date=29 March 2022 |website=Centers for Disease Control and Prevention |language=en-us}}</ref> The disease often spreads in [[Disease cluster|clusters]], where infections can be traced back to an index case or geographical location.<ref name="Lui 2020">{{cite journal | vauthors = Liu T, Gong D, Xiao J, Hu J, He G, Rong Z, Ma W | title = Cluster infections play important roles in the rapid evolution of COVID-19 transmission: A systematic review | journal = International Journal of Infectious Diseases | volume = 99 | pages = 374–380 | date = October 2020 | pmid = 32768702 | pmc = 7405860 | doi = 10.1016/j.ijid.2020.07.073 }}</ref> Often in these instances, [[superspreading event]]s occur, where many people are infected by one person.<ref name="AnnalsTransmit2" />

A person can get COVID-19 [[Transmission (medicine)#Indirect contact|indirectly]] by touching a [[Fomite|contaminated surface or object]] before touching their own mouth, nose, or eyes,<ref name="CDNASoNG" /><ref name="who_implications">{{cite web|date=9 July 2020|title=Transmission of SARS-CoV-2: implications for infection prevention precautions|url=https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions|publisher=World Health Organization|format=PDF}}</ref> though strong evidence suggests this does not contribute substantially to new infections.<ref name="WHO-ECDC-CDC-UK-Ca-Aus-Annals" /> Transmission from human to animal is possible, as in the first case, but the probability of a human [[vector (epidemiology)|contracting the disease from an animal]] is considered very low.<ref>{{cite web|url=https://www.cdc.gov/coronavirus/2019-ncov/daily-life-coping/animals.html|title=COVID-19 and Your Health|date=11 February 2020|website=Centers for Disease Control and Prevention}}</ref> Although it is considered possible, there is no direct evidence of the virus being transmitted by skin to skin contact.<ref name="AnnalsTransmit2" /> Transmission through [[Human feces|feces]] and [[wastewater]] have also been identified as possible.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2021/03/210301133628.htm|title=Transmission risk of COVID-19 from sewage spills into rivers can now be quickly quantified|website=ScienceDaily}}</ref> The virus is not known to spread through [[urine]], [[breast milk]], [[Foodborne illness|food]], or [[drinking water]].<ref name="who_implications" /><ref name="who_sanitation">{{cite web|date=29 July 2020|title=Water, sanitation, hygiene, and waste management for SARS-CoV-2, the virus that causes COVID-19|url=https://apps.who.int/iris/rest/bitstreams/1292822/retrieve|access-date=14 October 2020|website=[[World Health Organization|www.who.int]]|format=PDF}}</ref> It very rarely transmits [[Vertical transmission|from mother to baby]] during pregnancy.<ref name="AnnalsTransmit2" />

==Infectious period==
{{COVID-19 pandemic sidebar}}
After people are infected with COVID-19, they are able to transmit the disease to other people beginning as early as four to five days before developing symptoms, known as presymptomatic transmission.<ref name="CDNASoNG" /> To reduce such transmission, [[contact tracing]] is used to find and alert people who have been in contact with an infected individual in the 48 to 72 hours before they develop symptoms, or before that individual's test date if asymptomatic.<ref name="CDNASoNG" /> Initial reports suggested that this early transmission was restricted to the two-to-three day time window,<ref>{{cite journal |last1=He |first1=Xi |last2=Lau |first2=Eric H. Y. |last3=Wu |first3=Peng |last4=Deng |first4=Xilong |last5=Wang |first5=Jian |last6=Hao |first6=Xinxin |last7=Lau |first7=Yiu Chung |last8=Wong |first8=Jessica Y. |last9=Guan |first9=Yujuan |last10=Tan |first10=Xinghua |last11=Mo |first11=Xiaoneng |last12=Chen |first12=Yanqing |last13=Liao |first13=Baolin |last14=Chen |first14=Weilie |last15=Hu |first15=Fengyu |last16=Zhang |first16=Qing |last17=Zhong |first17=Mingqiu |last18=Wu |first18=Yanrong |last19=Zhao |first19=Lingzhai |last20=Zhang |first20=Fuchun |last21=Cowling |first21=Benjamin J. |last22=Li |first22=Fang |last23=Leung |first23=Gabriel M. |title=Temporal dynamics in viral shedding and transmissibility of COVID-19 |journal=Nature Medicine |date=1 May 2020 |volume=26 |issue=5 |pages=672–675 |doi=10.1038/s41591-020-0869-5|pmid=32296168 |s2cid=215761082 |doi-access=free }}</ref> but an author correction later acknowledged that transmission could begin four to five days before symptom onset.<ref name=HeCorrection/>

People are most infectious shortly before and after their symptoms begin<ref name=HeCorrection/>—even if mild or non-specific—as the [[viral load]] peaks at this time.<ref name="CDNASoNG" /><ref name="who_implications" />

Based on current evidence, adults with mild to moderate COVID-19 remain infectious (i.e., shed replication-competent SARS-CoV-2) for up to ten days after symptoms begin, although there are few transmission events are observed after five days.<ref name=HeCorrection/> Adults with severe to critical COVID-19, or severe immune suppression (immunocompromised persons), may remain infectious (i.e., shed replication-competent SARS-CoV-2) for up to 20 days after symptoms begin.<ref name="ECDCQA">{{cite web|title=Q & A on COVID-19: Basic facts|url=https://www.ecdc.europa.eu/en/covid-19/questions-answers/questions-answers-basic-facts|date=21 September 2021|publisher=[[European Centre for Disease Prevention and Control]]}}</ref><ref name=CDCClinicalQuestions/>

Patients who are tested positive to the virus again after recovery, in case they weren't being reinfected, is found to be not transmitting the virus to others.<ref>{{cite web|url=https://science.thewire.in/health/coronavirus-reinfection-risk-hong-kong-study/|title=What We Really Know About the Risk of Coronavirus Reinfection – The Wire Science|first=Angela|last=Rasmussen|date=4 September 2020 }}</ref>

Nearly a third of people with COVID-19 remain contagious five days after the onset of symptoms or a positive test. This is reduced to 7% for those who test negative twice with rapid tests on days 5 and 6. Without testing, 5% are contagious on day 10.<ref>{{cite news |title=One in seven could still be infectious after five-day Covid isolation |url=https://www.theguardian.com/world/2022/jan/12/still-infectious-five-day-covid-isolation |access-date=5 June 2022 |work=The Guardian |date=12 January 2022}}</ref><ref>{{cite news |title=Here's how long people with COVID-19 might remain contagious, according to the best available data |url=https://www.businessinsider.com/covid-how-long-contagious-infectious-period-omicron-delta-rapid-test-2022-1 |access-date=5 June 2022 |work=Business Insider |date=22 January 2022}}</ref>

=== Asymptomatic transmission ===
[[File:Asymptomatic COVID by age.jpg|thumb|left|Proportion of asymptomatic SARS-CoV-2 infection by age. About 44% of those infected with SARS-CoV-2 remained asymptomatic throughout the infection.<ref name="Wang2023"/>]]

People who are [[asymptomatic]] do not show symptoms but still are able to transmit the virus.<ref name="WHO-ECDC-CDC-UK-Ca-Aus-Annals"/> At least a third of the people who are infected with the virus do not develop noticeable symptoms at any point in time.<ref name="Wang2023">{{cite journal |last1=Wang |first1=Bing |last2=Andraweera |first2=Prabha |last3=Elliott |first3=Salenna |last4=Mohammed |first4=Hassen |last5=Lassi |first5=Zohra |last6=Twigger |first6=Ashley |last7=Borgas |first7=Chloe |last8=Gunasekera |first8=Shehani |last9=Ladhani |first9=Shamez |last10=Marshall |first10=Helen Siobhan |title=Asymptomatic SARS-CoV-2 Infection by Age: A Global Systematic Review and Meta-analysis |journal=The Pediatric Infectious Disease Journal |date=March 2023 |volume=42 |issue=3 |pages=232–239 |doi=10.1097/INF.0000000000003791 |doi-access=free |pmid=36730054 |pmc=9935239 }}</ref><ref>Multiple sources:
* {{cite journal | vauthors = Oran DP, Topol EJ | title = The Proportion of SARS-CoV-2 Infections That Are Asymptomatic : A Systematic Review | journal = Annals of Internal Medicine | volume = 174 | issue = 5 | pages = 655–662 | date = May 2021 | pmid = 33481642 | pmc = 7839426 | doi = 10.7326/M20-6976 }}
* {{cite web|title=Transmission of COVID-19|url=https://www.ecdc.europa.eu/en/covid-19/latest-evidence/transmission|access-date=6 December 2020|website=European Centre for Disease Prevention and Control}}
* {{cite journal | vauthors = Nogrady B | title = What the data say about asymptomatic COVID infections | journal = Nature | volume = 587 | issue = 7835 | pages = 534–535 | date = November 2020 | pmid = 33214725 | doi = 10.1038/d41586-020-03141-3 | doi-access = free | bibcode = 2020Natur.587..534N }}</ref><ref name="Asymptomatic">{{cite journal | vauthors = Gao Z, Xu Y, Sun C, Wang X, Guo Y, Qiu S, Ma K | title = A systematic review of asymptomatic infections with COVID-19 | journal = Journal of Microbiology, Immunology, and Infection = Wei Mian Yu Gan Ran Za Zhi | volume = 54 | issue = 1 | pages = 12–16 | date = February 2021 | pmid = 32425996 | pmc = 7227597 | doi = 10.1016/j.jmii.2020.05.001 | doi-access = free }}</ref> Asymptomatic carriers tend not to get tested.<ref name="Asymptomatic" /><ref>{{cite journal | vauthors = Oran DP, Topol EJ | title = Prevalence of Asymptomatic SARS-CoV-2 Infection : A Narrative Review | journal = Annals of Internal Medicine | volume = 173 | issue = 5 | pages = 362–367 | date = September 2020 | pmid = 32491919 | pmc = 7281624 | doi = 10.7326/M20-3012 }}</ref><ref>{{cite journal | vauthors = Lai CC, Liu YH, Wang CY, Wang YH, Hsueh SC, Yen MY, Ko WC, Hsueh PR | title = Asymptomatic carrier state, acute respiratory disease, and pneumonia due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2): Facts and myths | journal = Journal of Microbiology, Immunology, and Infection = Wei Mian Yu Gan Ran Za Zhi | volume = 53 | issue = 3 | pages = 404–412 | date = June 2020 | pmid = 32173241 | pmc = 7128959 | doi = 10.1016/j.jmii.2020.02.012 }}</ref>

Persons with asymptomatic COVID-19 infection can have the same viral load as symptomatic and presymptomatic cases, and are able to transmit the virus.<ref name="CDNASoNG"/> However, the infectious period of asymptomatic cases has been observed to be shorter with faster viral clearance.<ref name="CDNASoNG"/>

==Dominant mode of transmission: airborne/aerosol==
[[File:Pass me the pepper please.ogg|thumb|Person's breath, shown here when speaking, forms roughly a cone-shaped plume of warm humid air, that breaks up into rolls.<ref name="Abkarian_2020">{{cite journal | vauthors = Abkarian M, Mendez S, Xue N, Yang F, Stone HA | title = Speech can produce jet-like transport relevant to asymptomatic spreading of virus | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 117 | issue = 41 | pages = 25237–25245 | date = October 2020 | pmid = 32978297 | pmc = 7568291 | doi = 10.1073/pnas.2012156117 | arxiv = 2006.10671 | bibcode = 2020PNAS..11725237A | doi-access = free }}</ref> The virus-containing droplets in the breath of an infected person, are carried out into the surroundings, by this plume (person speaking on right hand side of screen).]]

The dominant mode of transmission of the COVID-19 virus is exposure to respiratory droplets (small liquid particles) carrying infectious virus (i.e., [[airborne transmission|airborne or aerosol transmission]]).<ref name="CDCScientificBriefTransmission">{{cite web|date=7 May 2021|title=Scientific Brief: SARS-CoV-2 Transmission|url=https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/sars-cov-2-transmission.html|access-date=8 May 2021|publisher=Centers for Disease Control and Prevention}}</ref><ref name="cdc_spread">{{cite web|title=How COVID-19 Spreads|url=https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-covid-spreads.html |date=14 July 2021|publisher=[[Centers for Disease Control and Prevention]]}}</ref><ref name="CDCFAQ">{{cite web|date=13 September 2021|title=COVID-19 Frequently Asked Questions|url=https://www.cdc.gov/coronavirus/2019-ncov/faq.html|publisher=Centers for Disease Control and Prevention}}</ref><ref name=FomiteTransmissionCDC>{{cite web |date=5 April 2021|title=Science Brief: SARS-CoV-2 and Surface (Fomite) Transmission for Indoor Community Environments|url=https://www.cdc.gov/coronavirus/2019-ncov/more/science-and-research/surface-transmission.html|publisher=Centers for Disease Control and Prevention}}</ref><ref>{{cite journal | vauthors = Samet JM, Prather K, Benjamin G, Lakdawala S, Lowe JM, Reingold A, Volckens J, Marr L | title = Airborne Transmission of SARS-CoV-2: What We Know | journal = Clinical Infectious Diseases | pages = 1924–1926 | date = January 2021 | volume = 73 | issue = 10 | pmid = 33458756 | pmc = 7929061 | doi = 10.1093/cid/ciab039 }}</ref><ref name="Greenhalgh_2021"/><ref name="who_transmitted">{{cite web|date=30 April 2021|title=Coronavirus disease (COVID-19): How is it transmitted?|url=https://www.who.int/news-room/q-a-detail/coronavirus-disease-covid-19-how-is-it-transmitted|publisher=[[World Health Organization]]}}</ref><ref name="gov.uk coronavirus-background">{{cite web|title=COVID-19: epidemiology, virology and clinical features|publisher=[[UK Health Security Agency]]|url=https://www.gov.uk/government/publications/wuhan-novel-coronavirus-background-information/wuhan-novel-coronavirus-epidemiology-virology-and-clinical-features|date=6 October 2021}}</ref> Spread occurs when the particles are emitted from the mouth or nose of an infected person when they breathe, cough, sneeze, talk, or sing.<ref name="who_transmitted" /><ref name="Santarpia_2021">{{cite journal | vauthors = Santarpia JL, Herrera VL, Rivera DN, Ratnesar-Shumate S, Reid SP, Ackerman DN, Denton PW, Martens JW, Fang Y, Conoan N, Callahan MV, Lawler JV, Brett-Major DM, Lowe JJ | title = The size and culturability of patient-generated SARS-CoV-2 aerosol | journal = Journal of Exposure Science & Environmental Epidemiology | pages = 706–711 | date = August 2021 | volume = 32 | issue = 5 | pmid = 34408261 | pmc = 8372686 | doi = 10.1038/s41370-021-00376-8 }}</ref><ref name="Bourouiba_2021">{{cite journal|vauthors=Bourouiba L|date=5 January 2021|title=The Fluid Dynamics of Disease Transmission|journal=Annual Review of Fluid Mechanics|volume=53|issue=1|pages=473–508|bibcode=2021AnRFM..53..473B|doi=10.1146/annurev-fluid-060220-113712|issn=0066-4189|doi-access=free|s2cid=225114407}}</ref> Human breath forms a roughly cone-shaped plume of air; in an infected person, the breath carries out the virus-containing droplets.<ref name="Bourouiba_2021" /><ref name="Abkarian_2020" /> So we expect the highest concentration of virus-containing droplets to be directly in front of an infected person, which suggests that the risk of transmission is greatest within three to six feet of the source of the infection.<ref name="CDCScientificBriefTransmission"/><ref name="AR2021" /> But breath contains many droplets that smaller than 100 micrometres in size, and these can stay suspended in the air for at least minutes and move across a room.<ref>{{cite journal|vauthors=de Oliveira PM, Mesquita LC, Gkantonas S, Giusti A, Mastorakos E|date=January 2021|title=Evolution of spray and aerosol from respiratory releases: theoretical estimates for insight on viral transmission|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=477|issue=2245|pages=20200584|bibcode=2021RSPSA.47700584D|doi=10.1098/rspa.2020.0584|pmc=7897643|pmid=33633490|doi-access=free}}</ref><ref name="Lednicky_2020">{{cite journal|vauthors=Lednicky JA, Lauzardo M, Fan ZH, Jutla A, Tilly TB, Gangwar M, Usmani M, Shankar SN, Mohamed K, Eiguren-Fernandez A, Stephenson CJ, Alam MM, Elbadry MA, Loeb JC, Subramaniam K, Waltzek TB, Cherabuddi K, Morris JG, Wu CY|date=November 2020|title=Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients|journal=International Journal of Infectious Diseases|volume=100|pages=476–482|doi=10.1016/j.ijid.2020.09.025|pmc=7493737|pmid=32949774}}</ref><ref name="Bourouiba_2021" /><ref>{{cite journal|vauthors=Balachandar S, Zaleski S, Soldati A, Ahmadi G, Bourouiba L|date=2020|title=Host-to-host airborne transmission as a multiphase flow problem for science-based social distance guidelines|journal=International Journal of Multiphase Flow|volume=132|pages=103439|arxiv=2008.06113|doi=10.1016/j.ijmultiphaseflow.2020.103439|pmc=7471834|bibcode=2020IJMF..13203439B }}</ref><ref>{{cite journal|vauthors=Netz RR|date=August 2020|title=Mechanisms of Airborne Infection via Evaporating and Sedimenting Droplets Produced by Speaking|journal=The Journal of Physical Chemistry B|volume=124|issue=33|pages=7093–7101|doi=10.1021/acs.jpcb.0c05229|pmc=7409921|pmid=32668904}}</ref> There is evidence that infectious SARS-CoV-2 survives in aerosols for a few hours.<ref name="Doremalen">{{cite journal|vauthors=van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, Tamin A, Harcourt JL, Thornburg NJ, Gerber SI, Lloyd-Smith JO, de Wit E, Munster VJ|date=April 2020|title=Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1|journal=The New England Journal of Medicine|volume=382|issue=16|pages=1564–1567|doi=10.1056/NEJMc2004973|pmc=7121658|pmid=32182409|doi-access=free}}</ref> There is substantial evidence for transmission events across a room (i.e., over distances larger than a metre or two) that is associated with being indoors, particularly in poorly ventilated spaces, although even indoor air drafts driven by air conditioning systems may contribute to the spread of respiratory sections.<ref name="Miller_2021" /><ref name=":0">{{cite journal|last=Hunziker|first=Patrick|date=1 October 2021|title=Minimising exposure to respiratory droplets, 'jet riders' and aerosols in air-conditioned hospital rooms by a 'Shield-and-Sink' strategy|url=https://bmjopen.bmj.com/content/11/10/e047772|journal=BMJ Open|volume=11|issue=10|pages=e047772|doi=10.1136/bmjopen-2020-047772|pmid=34642190|pmc=8520596|issn=2044-6055}}</ref><ref name="Li_2021">{{cite journal|vauthors=Li Y, Qian H, Hang J, Chen X, Cheng P, Ling H, Wang S, Liang P, Li J, Xiao S, Wei J, Liu L, Cowling BJ, Kang M|date=June 2021|title=Probable airborne transmission of SARS-CoV-2 in a poorly ventilated restaurant|journal=Building and Environment|volume=196|pages=107788|doi=10.1016/j.buildenv.2021.107788|pmc=7954773|pmid=33746341|bibcode=2021BuEnv.19607788L }}</ref> This has led to statements that transmission occurs most easily in the "three C's": crowded places, close contact settings, and confined and enclosed spaces.<ref name="who_transmitted" />
[[File:Safe Spaces - how to reduce virus transmission indoors - 9hP41EaypD4.webm|thumb|thumbtime=6:42|Reducing airborne transmission of COVID-19 indoors (video)]]
This mode of transmission occurs via an infected person breathing out the virus, which is then carried by the air to a person nearby, or to someone across a room, who then breathes the virus in. Attempts to reduce airborne transmission act on one or more of these steps in transmission.<ref name="Prather_2020">{{cite journal | vauthors = Prather KA, Wang CC, Schooley RT | title = Reducing transmission of SARS-CoV-2 | language = EN | journal = Science | volume = 368 | issue = 6498 | pages = 1422–1424 | date = June 2020 | pmid = 32461212 | doi = 10.1126/science.abc6197 | bibcode = 2020Sci...368.1422P | doi-access = free }}</ref> [[Face masks during the COVID-19 pandemic|Masks or face coverings]] are worn to reduce the virus breathed out by an infected person (who may not know they are infected), as well as the virus breathed in by a susceptible person. [[Social distancing]] keeps people apart. To prevent virus building up in the air of a room occupied by one or more infected people,<ref name="Prather_2020" /> [[Ventilation (architecture)|ventilation]] is used to vent virus-laden air to the outside (where it will be diluted in the atmosphere) and replace it with virus-free air from the outside. Alternatively, the air may passed through [[Filtration|filters]] to remove the virus-containing particles. A combination of shielding (protection from large droplet ejection) and air filtering, eliminating aerosols, ("Shield and sink" strategy) is particularly effective in reducing transfer of respiratory materials in indoor settings.<ref>{{cite journal|last=Hunziker|first=Patrick|date=1 October 2021|title=Minimising exposure to respiratory droplets, 'jet riders' and aerosols in air-conditioned hospital rooms by a 'Shield-and-Sink' strategy|url=https://bmjopen.bmj.com/content/11/10/e047772|journal=BMJ Open|volume=11|issue=10|pages=e047772|doi=10.1136/bmjopen-2020-047772|issn=2044-6055|pmid=34642190|pmc=8520596}}</ref>

The sneeze resembles a free [[Turbulence|turbulent]] [[Jet stream|jet]]. The turbulent multiphase cloud contributes critically to increasing the range of the [[pathogen]]-bearing drops originating in human [[Coughs and sneezes spread diseases|coughs]] and [[sneeze]]s.<ref name=":2">{{cite journal |last1=Bourouiba |first1=Lydia |last2=Dehandschoewercker |first2=Eline |last3=Bush |first3=John W. M. |date=24 March 2014 |title=Violent expiratory events: on coughing and sneezing |url=|journal=Journal of Fluid Mechanics |volume=745 |pages=537–563 |doi=10.1017/jfm.2014.88 |bibcode=2014JFM...745..537B |hdl=1721.1/101386 |s2cid=2102358 |issn=0022-1120|hdl-access=free }}</ref>  The jet's reach is nearly 22&nbsp;ft in 18.5 seconds and 25&nbsp;ft in 22 seconds.<ref name=":3">{{cite journal |last1=Arumuru |first1=Venugopal |last2=Pasa |first2=Jangyadatta |last3=Samantaray |first3=Sidhartha Sankar |date=1 November 2020 |title=Experimental visualization of sneezing and efficacy of face masks and shields |url=|journal=Physics of Fluids |volume=32 |issue=11 |pages=115129 |doi=10.1063/5.0030101 |pmid=33244217 |pmc=7684680 |bibcode=2020PhFl...32k5129A |issn=1070-6631}}</ref> The shape of the expelled particles is conical, with a spreading angle of 23 degrees.<ref name=":3" /><ref>{{cite journal |last1=Bahl |first1=Prateek |last2=de Silva |first2=Charitha |last3=MacIntyre |first3=C. Raina |last4=Bhattacharjee |first4=Shovon |last5=Chughtai |first5=Abrar Ahmad |last6=Doolan |first6=Con |date=2021-11-02 |title=Flow dynamics of droplets expelled during sneezing |url=|journal=Physics of Fluids |volume=33 |issue=11 |pages=111901 |doi=10.1063/5.0067609 |pmid=34803362 |pmc=8597717 |bibcode=2021PhFl...33k1901B |issn=1070-6631}}</ref> The trajectory of the turbulent jet is inclined due to the inclination angle of the nose.<ref name=":3" /> Smaller droplets travel a considerable distance as freely suspended tracers and may still get reflected and follow the turbulent cloud.<ref name=":3" /> Droplets with a diameter less than 50 μm remain suspended in the [[cloud]] for an extended period of time, which allows the cloud to reach heights of 4 to 6 meters, where [[Ventilation (architecture)|ventilation]] systems can be contaminated.<ref name=":2" />

Because [[physical intimacy]] and [[Human sexual activity|sex]] involve close contact, in October 2021, New York City Department of Health discouraged unvaccinated persons, immunocompromised people, people over 65, persons with COVID-19, people with a health condition that increases the risk of severe COVID-19, and people who live with someone from one of these groups from engaging in kissing, [[casual sex]], or other activities, and recommended wearing face mask during sex.<ref name="nyc_sex" />

The risk of transmission from all size droplets and aerosols is lower in indoor spaces with good ventilation.<ref name="Iillo">{{cite journal |department=Editorial |title=COVID-19 transmission-up in the air |journal=The Lancet Respiratory Medicine |volume=8 |issue=12 |page=1159 |date=December 2020 |pmid=33129420 |pmc=7598535 |doi=10.1016/s2213-2600(20)30514-2 |last1=The Lancet Respiratory Medicine}}</ref> The risk of outdoor transmission is low.<ref>{{cite journal |author1=Tommaso Celeste Bulfone |author2=Mohsen Malekinejad |author3=George W. Rutherford |author4=Nooshin Razani|title=Outdoor Transmission of SARS-CoV-2 and Other Respiratory Viruses: A Systematic Review|journal=Journal of Infectious Diseases|volume=223|issue=4|date=15 February 2021|pages=550–561|doi=10.1093/infdis/jiaa742 |pmid=33249484|pmc=7798940 |url=https://academic.oup.com/jid/article/223/4/550/6009483}}</ref><ref>{{cite web|title=Participate in Outdoor and Indoor Activities|publisher=U.S. Centers for Disease Control and Prevention|date=19 August 2021|url=https://www.cdc.gov/coronavirus/2019-ncov/daily-life-coping/outdoor-activities.html}}</ref>

Transmission events occur in workplaces, schools, conferences, sporting venues, dormitories, prisons, shopping facilities, and ships,<ref name="BZcaB">{{cite journal |vauthors=Leclerc QJ, Fuller NM, Knight LE, Funk S, Knight GM |title=What settings have been linked to SARS-CoV-2 transmission clusters? |journal=Wellcome Open Research |volume=5 |page=83 |date=5 June 2020 |pmid=32656368 |pmc=7327724 |doi=10.12688/wellcomeopenres.15889.2 |doi-access=free |first5=Sebastian |first6=Gwenan M.}}</ref> as well as restaurants,<ref name="Li_2021" /> passenger vehicles,<ref>{{cite journal |author1=Varghese Mathai |author2=Asimanshu Das |author3=Jeffrey A. Bailey |author4=Kenneth Breuer |date=1 January 2021 |volume=7 |issue=1 |title=Airflows inside passenger cars and implications for airborne disease transmission |journal=Science Advances |doi=10.1126/sciadv.abe0166 |pmid=33277325 |pmc=7775778 |arxiv=2007.03612 |bibcode=2021SciA....7..166M}}</ref> religious buildings and choir practices,<ref>{{cite journal |vauthors=Katelaris AL, Wells J, Clark P, Norton S, Rockett R, Arnott A, Sintchenko V, Corbett S, Bag SK |title=Epidemiologic Evidence for Airborne Transmission of SARS-CoV-2 during Church Singing, Australia, 2020 |journal=Emerging Infectious Diseases |volume=27 |issue=6 |pages=1677–1680 |date=June 2021 |pmid=33818372 |pmc=8153858 |doi=10.3201/eid2706.210465 |issn=1080-6040}}</ref> and hospitals and other healthcare settings.<ref name="long_distance"/> A [[superspreading event]] in a [[Skagit County, Washington]], choral practice resulted in 32 to 52 of the 61 attendees infected.<ref>{{cite journal |vauthors=Hamner L, Dubbel P, Capron I, Ross A, Jordan A, Lee J, Lynn J, Ball A, Narwal S, Russell S, Patrick D, Leibrand H |title=High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice - Skagit County, Washington, March 2020 |journal=MMWR. Morbidity and Mortality Weekly Report |volume=69 |issue=19 |pages=606–610 |date=May 2020 |pmid=32407303 |doi=10.15585/mmwr.mm6919e6 |doi-access=free}}</ref><ref name="Miller_2021"/>

An existing model of airborne transmission (the [[Wells-Riley model]]) was adapted to help understand why crowded and poorly ventilated spaces promote transmission,<ref name="Miller_2021" /> with findings supported by aerodynamic analysis of droplet transfer in air-conditioned hospital rooms.<ref name=":0" /> Airborne transmission also occurs in healthcare settings; long-distance dispersal of virus particles has been detected in ventilation systems of a hospital.<ref name="long_distance">{{cite journal | vauthors = Nissen K, Krambrich J, Akaberi D, Hoffman T, Ling J, Lundkvist Å, Svensson L, Salaneck E | title = Long-distance airborne dispersal of SARS-CoV-2 in COVID-19 wards | journal = Scientific Reports | volume = 10 | issue = 1 | pages = 19589 | date = November 2020 | pmid = 33177563 | pmc = 7659316 | doi = 10.1038/s41598-020-76442-2 | bibcode = 2020NatSR..1019589N }}</ref>

Some scientists criticized public health authorities, [[World Health Organization's response to the COVID-19 pandemic#Reception|including the WHO]], in 2020 for being too slow to recognize airborne (aerosol) transmission of COVID-19 and to update their public health guidance accordingly.<ref name="nature_airborne">{{cite journal | vauthors = Lewis D | title = Mounting evidence suggests coronavirus is airborne - but health advice has not caught up | journal = Nature | volume = 583 | issue = 7817 | pages = 510–513 | date = July 2020 | pmid = 32647382 | doi = 10.1038/d41586-020-02058-1 | bibcode = 2020Natur.583..510L | s2cid = 220470431 | doi-access = free }}</ref><ref name="rgyr7">{{cite journal | vauthors = Zhang R, Li Y, Zhang AL, Wang Y, Molina MJ | title = Identifying airborne transmission as the dominant route for the spread of COVID-19 | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 117 | issue = 26 | pages = 14857–14863 | date = June 2020 | pmid = 32527856 | pmc = 7334447 | doi = 10.1073/pnas.2009637117 | bibcode = 2020PNAS..11714857Z | doi-access = free }}</ref><ref name="dEjc5">{{cite journal | vauthors = Tanne JH | title = Covid-19: CDC publishes then withdraws information on aerosol transmission | journal = BMJ | volume = 370 | pages = m3739 | date = September 2020 | pmid = 32973037 | doi = 10.1136/bmj.m3739 | s2cid = 221881893 | doi-access = free }}</ref><ref name="10.1038/d41586-022-00925-7">{{cite journal |last1=Lewis |first1=Dyani |title=Why the WHO took two years to say COVID is airborne |journal=Nature |date=6 April 2022 |volume=604 |issue=7904 |pages=26–31 |language=en |doi=10.1038/d41586-022-00925-7|pmid=35388203 |bibcode=2022Natur.604...26L |s2cid=248000902 |doi-access=free }}</ref> By mid-2020, some public health authorities had updated their guidance to reflect the importance of airborne transmission.<ref name="CDCScientificBriefTransmission"/><ref name="PHACanadaTransmit">{{cite web|date=3 November 2020|title=COVID-19: Main modes of transmission|url=https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/health-professionals/main-modes-transmission.html|access-date=25 November 2020|work=Public Health Agency of Canada}}</ref> The WHO updated it only by 23 December 2021.<ref name="10.1038/d41586-022-00925-7"/><ref name="who_transmitted" />

=== Medical procedures designated as aerosol-generating procedures ===
There is concern that some medical procedures that affect the mouth and lungs can also generate aerosols, and that this may increase the infection risk. Some medical procedures have been designated as [[aerosol-generating procedure]]s (AGPs),<ref name="who_transmitted" /><ref name="pmid22563403">{{cite journal | vauthors = Tran K, Cimon K, Severn M, Pessoa-Silva CL, Conly J | title = Aerosol generating procedures and risk of transmission of acute respiratory infections to healthcare workers: a systematic review | journal = PLOS ONE | volume = 7 | issue = 4 | pages = e35797 | year = 2012 | pmid = 22563403 | pmc = 3338532 | doi = 10.1371/journal.pone.0035797 | doi-access = free | bibcode = 2012PLoSO...735797T }}</ref> but this been done without measuring the [[aerosols]] these procedures produce.<ref name="Hamilton_2021">{{cite journal | vauthors = Hamilton F, Arnold D, Bzdek BR, Dodd J, Reid J, Maskell N | title = Aerosol generating procedures: are they of relevance for transmission of SARS-CoV-2? | journal = The Lancet. Respiratory Medicine | volume = 9 | issue = 7 | pages = 687–689 | date = July 2021 | pmid = 33965002 | pmc = 8102043 | doi = 10.1016/S2213-2600(21)00216-2 }}</ref> The aerosols generated by some AGPs have been measured and found to be less than the aerosols produced by breathing.<ref>{{cite journal | vauthors = Wilson NM, Marks GB, Eckhardt A, Clarke AM, Young FP, Garden FL, Stewart W, Cook TM, Tovey ER | title = The effect of respiratory activity, non-invasive respiratory support and facemasks on aerosol generation and its relevance to COVID-19 | journal = Anaesthesia | volume = 76 | issue = 11 | pages = 1465–1474 | date = November 2021 | pmid = 33784793 | pmc = 8250912 | doi = 10.1111/anae.15475 | doi-access = free }}</ref> Less virus (strictly speaking, viral RNA){{efn|name=RNA|Viral RNA is much easier to detect and quantify than counting the live virus. The results make for weaker evidence, however, as inactive viruses still contain detectable levels of RNA. For airborne studies where the transmission is otherwise confirmed, RNA would be an acceptable surrogate for viral load; for paths that are not confirmed, however, RNA is not as convincing.}} has been found in the air near [[intensive care unit]] (ICUs) with COVID-19 patients than near rooms with COVID-19 patients that are not ICUs.<ref>{{cite journal | vauthors = Grimalt JO, Vílchez H, Fraile-Ribot PA, Marco E, Campins A, Orfila J, van Drooge BL, Fanjul F | title = Spread of SARS-CoV-2 in hospital areas | journal = Environmental Research | volume = 204 | issue = Pt B | pages = 112074 | date = September 2021 | pmid = 34547251 | pmc = 8450143 | doi = 10.1016/j.envres.2021.112074 | issn=0013-9351 }}</ref> Patients in ICUs are more likely to be subject to mechanically ventilation, an AGP. This suggests that in hospitals, areas near ICUs may actually pose less risk of infection via aerosols. This has led to calls to reconsider AGPs.<ref name="Hamilton_2021" /> The WHO recommends the use of filtering facepiece respirators such as N95 or [[FFP mask|FFP2 masks]] in settings where aerosol-generating procedures are performed,<ref name="who_implications" /> while the U.S. CDC and the [[European Centre for Disease Prevention and Control]] recommend these controls in all situations related to COVID-19 patient treatment (other than during crisis shortages).<ref name="uGF02">{{cite web|url=https://www.ecdc.europa.eu/sites/default/files/documents/Infection-prevention-and-control-in-healthcare-settings-COVID-19_5th_update.pdf|title=Infection prevention and control and preparedness for COVID-19 in healthcare settings - fifth update}}</ref><ref name="PlsXd">{{cite web|title=Respiratory Protection During Outbreaks: Respirators versus Surgical Masks {{!}} {{!}} Blogs {{!}} CDC|date=9 April 2020 |url=https://blogs.cdc.gov/niosh-science-blog/2020/04/09/masks-v-respirators/|access-date=25 November 2020}}</ref><ref name="0Udxc">{{cite web|last=CDC|date=11 February 2020|title=Coronavirus Disease 2019 (COVID-19)|url=https://www.cdc.gov/coronavirus/2019-ncov/hcp/infection-control-recommendations.html|access-date=29 November 2020|website=Centers for Disease Control and Prevention}}</ref>

There is a research that suggests that variation in [[airway resistance]], as measured by [[Computational fluid dynamics|CFD (Computational Fluid Dynamics)]], may be a useful tool for predicting the forecast of [[critically ill]] COVID-19 patients.<ref>{{cite journal |last1=Pan |first1=Shi-yu |last2=Ding |first2=Ming |last3=Huang |first3=Jing |last4=Cai |first4=Yan |last5=Huang |first5=Ying-zi |date=September 2021 |title=Airway resistance variation correlates with prognosis of critically ill COVID-19 patients: A computational fluid dynamics study |url=|journal=Computer Methods and Programs in Biomedicine |volume=208 |pages=106257 |doi=10.1016/j.cmpb.2021.106257 |pmid=34245951 |pmc=8231702 |issn=0169-2607}}</ref>

==Rarer modes of transmission==
===Surface (fomite) transmission===
[[File:Door Handle.JPG|alt=A silver-colored door handle on a white door|thumb|Surfaces that are often touched such as [[door handle]]s may transmit COVID-19, although is not thought to be the main way the virus spreads.]]
A person can get COVID-19 by touching a surface or object that has the virus on it (called a [[fomite]]), and then touching their own mouth, nose, or eyes, but it is not the main mode of transmission, and the risk of surface transmission is low.<ref name=FomiteTransmissionCDC/><ref name="who_transmitted" /><ref name="AnnalsTransmit2" /><ref name="who_implications" /><ref name="ECDCQA" /><ref name="cdc_spread"/> As of July 2020, "no specific reports which have directly demonstrated fomite transmission" although "People who come into contact with potentially infectious surfaces often also have close contact with the infectious person, making the distinction between respiratory droplet and fomite transmission difficult to discern."<ref name="who_implications" />

Each contact with a surface contaminated with SARS-CoV-2 has less than a 1 in 10,000 chance of causing an infection.<ref name=FomiteTransmissionCDC/> Various surface survival studies have found no detectable viable virus on porous surfaces within minutes to hours, but have found viable virus persisting on non-porous surfaces for days to weeks.<ref name=FomiteTransmissionCDC/><ref name="who_implications" /> However, surface-survival studies do not reflect real-world conditions, which are less favorable to the virus.<ref name=FomiteTransmissionCDC/> Ventilation and changes in environmental conditions can kill or degrade the virus.<ref name="who_implications" /><ref name=FomiteTransmissionCDC/> For example, [[temperature]], [[humidity]], and [[ultraviolet radiation]] ([[sunlight]]) all influence reductions in viral viability and infectiousness on surfaces.<ref name="CDCScientificBriefTransmission"/> Fomite transmission risk is also reduced because the virus does not transfer efficiently from the surface to the hands, and then from the hands to the mucous membranes (mouth, nose, and eye).<ref name=FomiteTransmissionCDC/>

The initial amount of virus on the surface (i.e., the viral load in respiratory droplets) also affects fomite transmission risk.<ref name=FomiteTransmissionCDC/> [[Hand washing]] and periodic surface cleaning impede indirect contact transmission through fomites.<ref name="who_transmitted" /><ref name="cdc_spread" /><ref name=FomiteTransmissionCDC/> Fomite transmission can be easily prevented with use of regular household cleaners or [[disinfection]].<ref name=FomiteTransmissionCDC/><ref name="who_transmitted" /><ref name="cdc_cleaning">{{cite web|date=27 May 2020|title=COVID-19: Cleaning And Disinfecting Your Home|url=https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/disinfecting-your-home.html|access-date=7 October 2020|website=[[Centers for Disease Control and Prevention|www.cdc.gov]]}}</ref> When surface survival data and factors affecting real-world transmission are considered, "the risk of fomite transmission after a person with COVID-19 has been in an indoor space is minor after 3 days (72 hours), regardless of when it was last cleaned."<ref name=FomiteTransmissionCDC/>

===Animal vectors===
Although the COVID-19 virus likely originated in [[bat]]s, the pandemic is sustained through human-to-human spread, and the risk of animal-to-human spread of COVID-19 is low.<ref name="cdc_animals">{{cite web|date=6 October 2021|title=COVID-19 and Animals|url=https://www.cdc.gov/coronavirus/2019-ncov/daily-life-coping/animals.html|publisher=[[Centers for Disease Control and Prevention|www.cdc.gov]]}}</ref><ref name="cdc_pets">{{cite web|title=COVID-19: If You Have Pets|url=https://www.cdc.gov/healthypets/covid-19/pets.html|date=29 June 2021|website=[[Centers for Disease Control and Prevention|www.cdc.gov]]}}</ref> COVID-19 infections in non-human animals have included [[companion animal]]s (e.g., domestic cats, dogs, and [[ferret]]s); zoo and animal sanctuary residents (e.g., [[big cat]]s, [[otter]]s, and non-human primates); [[mink]] in [[mink farm]]s in multiple countries; and wild [[white-tailed deer]] in numerous [[U.S. state]]s.<ref name="cdc_animals"/> Most animal infections came after the animals were in close contact with a human with COVID-19, such as an owner or caretaker.<ref name="cdc_animals"/> Experimental research in laboratory settings also shows that other types of mammals (e.g., voles, rabbits, hamsters, pigs, [[macaque]]s, [[baboon]]s) can become infected.<ref name="cdc_animals"/> By contrast, [[chicken]]s and [[duck]]s do not seem to become infected with, or spread, the virus.<ref name="cdc_animals" /> There is no evidence that the COVID-19 virus can spread to humans from the skin, fur, or hair of pets.<ref name="cdc_pets" />  The U.S. CDC recommended that pet owners limit their pet's interactions with unvaccinated people outside their household; advises pet owners not to put face coverings on pets, as it could harm them; and states that pets should not be disinfected with cleaning products not approved for animal use.<ref name="cdc_pets" /> If a pet becomes sick with COVID-19, the CDC recommends that owners "follow similar recommended precautions as for people caring for an infected person at home."<ref name="cdc_pets" />

People sick with COVID-19 should avoid contact with pets and other animals, in the same manner that people sick with COVID-19 should avoid contact with people.<ref name="cdc_pets" />

==Vectors for which there is no evidence of COVID-19 transmission==
=== Mother to child ===
The is no evidence for [[Vertically transmitted infection|intrauterine transmission]] of COVID-19 from pregnant women to their fetuses.<ref name="who_implications" /> Studies have not found any viable virus in breast milk.<ref name="who_implications" /> [[Breast milk]] is unlikely to spread the COVID-19 virus to babies.<ref name="CDCBreastfeed">{{cite web|title=Breastfeeding and Caring for Newborns if You Have COVID-19 |url=https://www.cdc.gov/coronavirus/2019-ncov/if-you-are-sick/pregnancy-breastfeeding.html |date=18 August 2021}}</ref><ref name="WHOBreastfeed">{{cite web |format=PDF |website=[[World Health Organization|www.who.int]] |title=Breastfeeding and COVID-19 |publisher=World Health Organization |url=https://apps.who.int/iris/rest/bitstreams/1283442/retrieve |access-date=18 September 2020 |date=23 June 2020 |archive-url=https://web.archive.org/web/20201022073949/https://apps.who.int/iris/rest/bitstreams/1283442/retrieve |archive-date=22 October 2020 |url-status=bot: unknown }}</ref> Noting the benefits of [[breastfeeding]], the WHO recommends that mothers with suspected or confirmed COVID-19 should be encouraged to initiate or continue to breastfeed, while taking proper infection prevention and control measures.<ref name="WHOBreastfeed"/><ref name="who_implications" />

===Food and water===
No evidence suggests that handling food or consuming food is associated with transmission of COVID-19.<ref name=ECDCQ&A>{{cite web|url=https://www.ecdc.europa.eu/en/covid-19/questions-answers/questions-answers-various |title=Questions and answers on COVID-19: Various|date=8 September 2021 |publisher=European Centre for Disease Prevention and Control}}</ref><ref name=CDCFoodSafety>{{cite web |url=https://www.cdc.gov/foodsafety/newsletter/food-safety-and-Coronavirus.html|title=Food Safety and Coronavirus Disease 2019 (COVID-19)|date=22 June 2020|publisher=U.S. Centers for Disease Control and Prevention}}</ref> The COVID-19 virus had poor survivability on surfaces;<ref name=ECDCQ&A/> less than 1 in 10,000 contacts with contaminated surfaces, including non-food-related surfaces, lead to infection.<ref name=FomiteTransmissionCDC/> As a result, the risk of spread from food products or packaging is very low.<ref name=CDCFoodSafety/> Public health authorities recommend that people follow practice good hygiene by [[hand-washing|washing hands with soap and water]] before preparing and consuming food.<ref name=ECDCQ&A/><ref name=CDCFoodSafety/>

The COVID-19 virus has not been detected in [[drinking water]].<ref name=CDCWater>{{cite web |url=https://www.cdc.gov/coronavirus/2019-ncov/php/water.html|title=Water and COVID-19 FAQs: Information about Drinking Water, Treated Recreational Water, and Wastewater|publisher=U.S. Centers for Disease Control and Prevention|date=23 April 2020}}</ref> Conventional [[water treatment]] (filtration and disinfection) inactivates or removes the virus.<ref name=CDCWater/> COVID-19 virus RNA is found in untreated wastewater,<ref name=CDCWater/><ref name="who_sanitation"/><ref name="z3Il2">{{cite journal |vauthors=Corpuz MV, Buonerba A, Vigliotta G, Zarra T, Ballesteros F, Campiglia P, Belgiorno V, Korshin G, Naddeo V |title=Viruses in wastewater: occurrence, abundance and detection methods |journal=The Science of the Total Environment |volume=745 |page=140910 |date=November 2020 |pmid=32758747 |pmc=7368910 |doi=10.1016/j.scitotenv.2020.140910 |doi-access=free |bibcode=2020ScTEn.74540910C}}</ref>{{efn|name=RNA}} but there is no evidence of COVID-19 transmission through exposure to untreated wastewater or sewerage systems.<ref name=CDCWater/> There is also no evidence that COVID-19 transmission to humans occurs through water in [[swimming pool]]s, [[hot tub]]s, or spas.<ref name=CDCWater/>

===Other===
While SARS-CoV-2 RNA has been detected in the urine and feces of some persons infected with COVID-19,{{efn|name=RNA}} there is no evidence of COVID-19 transmission through feces or urine.<ref name="who_implications" /><ref name=CDCWater/> COVID-19 is not an [[insect-borne disease]]; there is also no evidence that [[mosquito]] are a [[disease vector|vector]] for COVID-19.<ref>{{cite journal|title=SARS-CoV-2 failure to infect or replicate in mosquitoes: an extreme challenge|date=17 July 2020|journal=Scientific Reports|author1=Yan-Jang S. Huang |author2=Dana L. Vanlandingham |author3=Ashley N. Bilyeu |author4=Haelea M. Sharp |author5=Susan M. Hettenbach |author6=Stephen Higgs|volume=10 |issue=1 |page=11915 |doi=10.1038/s41598-020-68882-7 |pmid=32681089 |pmc=7368071 |bibcode=2020NatSR..1011915H}}</ref> COVID-19 is not a [[sexually transmitted infection]]; while the virus has been found in the [[semen]] of people who have COVID-19, there is no evidence that the virus spreads through semen or [[vaginal fluid]],<ref name="nyc_sex">{{cite web|date=18 June 2021|title=Safer Sex and COVID-19 |url=https://www1.nyc.gov/assets/doh/downloads/pdf/imm/covid-sex-guidance.pdf |publisher=[[New York City Department of Health]]}}</ref> however transmission during [[sexual activities]] is still possible due to proximity during intimate activities which enable transmission through other paths.<ref>{{cite web|url=https://www.umms.org/coronavirus/what-to-know/managing-medical-conditions/coronavirus-risk/sex|title=Sex and Coronavirus |website=www.umms.org}}</ref>

== Transmission rate, patterns, clusters ==
Many people do not transmit the virus, but some transmit to many people, and the virus is considered to be "over dispersed"&nbsp;– the transmission rate has high heterogeneity.<ref name="AnnalsTransmit2" /><ref name="EdwN9">{{cite journal|vauthors=Endo A, Abbott S, Kucharski AJ, Funk S|date=2020 |title=Estimating the overdispersion in COVID-19 transmission using outbreak sizes outside China |journal=Wellcome Open Research|volume=5|pages=67 |doi=10.12688/wellcomeopenres.15842.3|pmc=7338915 |pmid=32685698 |doi-access=free }}</ref> "[[Superspreader|Super-spreading events]]" occur from this minority of infected people, generally indoors and usually in high-risk venues where people remain in close proximity and poor ventilation for an extended period, such as restaurants, nightclubs, and places of worship.<ref name="AnnalsTransmit2" /><ref name="KM_aerosol">{{cite journal|vauthors=Kohanski MA, Lo LJ, Waring MS |date=October 2020|title=Review of indoor aerosol generation, transport, and control in the context of COVID-19|journal=[[International Forum of Allergy & Rhinology]]|volume=10|issue=10|pages=1173–1179 |doi=10.1002/alr.22661|pmc=7405119|pmid=32652898}}</ref> Such crowded conditions enable the virus to spread easily via aerosols,<ref name="who_transmitted" /> they can create clusters of cases, where infections can be traced back to an [[index case]] or geographical location.<ref name="Lui 2020" />  Another important site for transmission is between members of the same household,<ref name="AnnalsTransmit2" /> as well as [[hospital]]s due to the abundance of [[Pathogens and Global Health|pathogens]] present.<ref name=":5">{{cite journal |last1=Peng |first1=Shanbi |last2=Chen |first2=Qikun |last3=Liu |first3=Enbin |date=April 2021 |title=Corrigendum to "The role of computational fluid dynamics tools on investigation of pathogen transmission: Prevention and control" [Sci. Total Environ. 746 (2020) 142090] |url=|journal=Science of the Total Environment |volume=764 |pages=142862 |doi=10.1016/j.scitotenv.2020.142862 |pmid=33138993 |pmc=8445364 |bibcode=2021ScTEn.76442862P |issn=0048-9697}}</ref> [[Vehicle|Traffic vehicles]] are also a site for transmission, since the control of the pathogen there is harder due to the weak [[Ventilation (architecture)|ventilation system]] and the high [[density]] of people.<ref name=":5" /> [[Emergency department]]s are also great sites for transmission of [[COVID-19]].<ref name=":6">{{cite journal |last1=Fawwaz Alrebi |first1=Odi |last2=Obeidat |first2=Bushra |last3=Atef Abdallah |first3=Ibrahim |last4=Darwish |first4=Eman F. |last5=Amhamed |first5=Abdulkarem |date=May 2022 |title=Airflow dynamics in an emergency department: A CFD simulation study to analyse COVID-19 dispersion |journal=Alexandria Engineering Journal |volume=61 |issue=5 |pages=3435–3445 |doi=10.1016/j.aej.2021.08.062 |s2cid=237319234 |issn=1110-0168|doi-access=free }}</ref> The dispersion of [[respiratory droplet]]s can be influenced by various factors, including the [[Ventilation (architecture)|ventilation system]], the number of [[Infection|infected patients]], and their movements, which highlights the importance of proper ventilation and [[Air filtration|air filtration systems]] in reducing the spread of [[COVID-19]] within an emergency department setting.<ref name=":6" />

COVID-19 is more infectious than [[influenza]], but less so than [[measles]].<ref name="cdc_spread" /> Estimates of the number of people infected by one person with COVID-19—the [[basic reproduction number]] (''R<sub>0</sub>'')—have varied. In November 2020, a systematic review estimated R<sub>0</sub> of the original Wuhan strain to be approximately {{Estimate|2.87|2.39|3.44}}.<ref>{{cite journal |vauthors=Billah MA, Miah MM, Khan MN|date=11 November 2020|title=Reproductive number of coronavirus: A systematic review and meta-analysis based on global level evidence|journal=PLOS ONE|volume=15|issue=11 |at=e0242128|bibcode=2020PLoSO..1542128B |doi=10.1371/journal.pone.0242128|pmc=7657547|pmid=33175914|doi-access=free}}</ref> The R<sub>0</sub> of the [[SARS-CoV-2 Delta variant|Delta variant]], which became the dominant variant of COVID-19 in 2021, is substantially higher. Among five studies catalogued in October 2021, Delta's mean estimate R<sub>0</sub> was 5.08.<ref name=":4">{{cite journal|title=The reproductive number of the Delta variant of SARS-CoV-2 is far higher compared to the ancestral SARS-CoV-2 virus |author1=Ying Liu|author2=Joacim Rocklöv|journal=Journal of Travel Medicine|volume=28|issue=7 |date=October 2021|doi=10.1093/jtm/taab124|pmid=34369565|pmc=8436367 |url=https://academic.oup.com/jtm/article/28/7/taab124/6346388}}</ref>
[[File:Ncase reproduction number.png|thumb|389x389px|Visualisation of the [[basic reproduction number]], R<sub>0</sub>]]
 

[[Temperature]] is also a factor that affects the [[Pathogen transmission|transmissibility]] of the [[virus]]. At elevated temperatures and low virus [[Concentration|concentration rates]] the virus is in its weak state<ref name=":7">{{cite journal |last1=Dbouk |first1=Talib |last2=Drikakis |first2=Dimitris |date=2021-02-01 |title=Fluid dynamics and epidemiology: Seasonality and transmission dynamics |url=|journal=Physics of Fluids |volume=33 |issue=2 |pages=021901 |doi=10.1063/5.0037640 |pmid=33746486 |pmc=7976049 |bibcode=2021PhFl...33b1901D |issn=1070-6631}}</ref> and the spreading of it is strenuously. At low temperatures and excessive virus concentration rates the virus is in its robust state<ref name=":7" /> and the spreading of it is unchallenged.

== Effect of face masks and face shields ==
{{main|Source control (respiratory disease)}}
[[Source control (respiratory disease)|Source control]] is the principal mode of protection of COVID-19<ref name=new_exhale>{{cite journal <!-- Citation bot bypass--> | vauthors = Hazard JM, Cappa CD | title = Performance of Valved Respirators to Reduce Emission of Respiratory Particles Generated by Speaking | journal = Environmental Science & Technology Letters | volume = 9 | issue = 6 | pages = 557–560 | date = June 2022 | pmid = 37552726 | doi = 10.1021/acs.estlett.2c00210 | bibcode = 2022EnSTL...9..557H }}</ref> after receiving the [[COVID-19 vaccine|vaccine]]. There are many types of face masks, including [[surgical mask]], two-layered face mask, [[face shield]], and [[N95 respirator]]. A surgical mask is the least effective means of preventing [[particle]] leakage since the leaked particles due to a [[sneeze]] travelling a distance of 2.5&nbsp;ft.<ref name=":3" /> The combination of a surgical mask with a face shield restricts the forward motion of particles notably.<ref name=":3" /> A two-layered face mask has noticeable leakage in the forward direction, but with the addition of a [[cotton]] stitch there is significantly less leakage of particles.<ref name=":3" /> The combination of a two-layered face mask and a face shield effectively restricts the leakage in the forward direction.<ref name=":3" /> The face shield enables particles to escape from below it, and thus it is not recommended for protecting the spreading of the [[virus]].<ref name=":3" /> An N95 respirator completely restricts the forward leakage of particles, but in a badly fitting respirator, a significant amount of particles escape through the gap between the nose and the mask.<ref name=fit>{{cite journal | doi=10.1371/journal.pone.0085299 | doi-access=free | title=Fitting Characteristics of N95 Filtering-Facepiece Respirators Used Widely in China | date=2014 | journal=PLOS ONE | volume=9 | issue=1 | pages=e85299 | pmid=24465528 | bibcode=2014PLoSO...985299Y | vauthors = Yu Y, Jiang L, Zhuang Z, Liu Y, Wang X, Liu J, Yang M, Chen W | pmc=3897424 }}</ref><ref name=":3" />
[[File:8210-Cropped.jpg|thumb|333x333px|[[N95 respirator]]]]
None of the protective face masks and face shields completely block the escape of particles projected by a [[sneeze]], but they all effectively reduce the leakage and reach of the sneeze within 1–3&nbsp;ft. The N95 respirator is the best face coverage for mitigating the spread because it completely impedes the forward leakage of the particles.<ref name=":3" /> The widely accepted safe distance of 6&nbsp;ft is highly underestimated for sneezing.<ref name=":3" /> Researchers strongly recommend using the elbow or hands to prevent droplet leakage even when wearing face masks during [[sneezing]] and [[coughing]].<ref name=":3" /> Wearing masks in indoor spaces reduces the risk of transmission,<ref>{{cite journal |last=Ho |first=Clifford K. |date=2021 |title=Modelling Airborne Transmission and Ventilation Impacts of a COVID-19 Outbreak in a Restaurant in Guangzhou, China |url=|journal=International Journal of Computational Fluid Dynamics |volume=35 |issue=9 |pages=708–726|doi=10.1080/10618562.2021.1910678 |bibcode=2021IJCFD..35..708H |osti=1778059 |s2cid=233596966 }}</ref> but it is recommended to immediately evacuate any space where sneezing has occurred.<ref name=":3" />

== Effect of vaccination ==
The [[Pfizer-BioNTech COVID-19 vaccine|Pfizer-BioNTech]], [[Moderna COVID-19 vaccine|Moderna]], [[Oxford–AstraZeneca COVID-19 vaccine|AstraZeneca]] and [[Janssen COVID-19 vaccine]]s provide effective protection against COVID-19, including against severe disease, hospitalisation, and death, and "a growing body of evidence suggests that [[COVID-19 vaccine]]s also reduce asymptomatic infection and transmission" as chains of transmission are interrupted by vaccines.<ref name=CDCScienceBriefVaccination/> While fully vaccinated people can still become infected and potentially transmit the virus to others (particularly in areas of widespread [[community transmission]]), they do so at a much lower rate than unvaccinated people.<ref name=CDCScienceBriefVaccination>{{cite web|title=Science Brief: COVID-19 Vaccines and Vaccination|url=https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/fully-vaccinated-people.html|date=15 September 2021|publisher=U.S. Centers for Disease Control and Prevention}}</ref> The primary cause of continued spread of COVID-19 is transmission between unvaccinated people.<ref name=CDCScienceBriefVaccination/>

== References ==
{{notelist}}
{{reflist}}

== External links ==
* [https://english.elpais.com/society/2020-10-28/a-room-a-bar-and-a-class-how-the-coronavirus-is-spread-through-the-air.html A room, a bar and a classroom] (visualization of how COVID-19 does and does not spread)

{{COVID-19 pandemic}}
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[[Category:COVID-19 pandemic]]
[[Category:Disease transmission]]
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