Transmission of COVID-19

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Video explainer on reducing airborne transmission of COVID-19 indoors

Transmission of COVID-19 is the passing from person to person of coronavirus disease 2019. The virus is transmitted mainly through the respiratory route after an infected person coughs, sneezes, sings, talks, or breathes.[1] A new infection occurs when virus-containing particles exhaled by an infected person, either respiratory droplets or aerosols, get into the mouth, nose, or eyes of other people who are in close contact with the infected person.[2][3][4][5] During human-to-human transmission, an average 1000 infectious SARS-CoV-2 virions are thought to initiate a new infection.[6][7]

The closer people interact, and the longer they interact, the more likely they are to transmit COVID-19. Closer distances can involve larger droplets (which fall to the ground) and aerosols, whereas longer distances only involve aerosols.[4] Larger droplets can also turn into aerosols (known as droplet nuclei) through evaporation.[8] The relative importance of the larger droplets and the aerosols is not clear as of November 2020; however, the virus is not known to spread between rooms over long distances such as through air ducts.[9] Airborne transmission is able to particularly occur indoors, in high risk locations[9] such as restaurants, choirs, gyms, nightclubs, offices, and religious venues, often when they are crowded or less ventilated.[8] It also occurs in healthcare settings, often when aerosol-generating medical procedures are performed on COVID-19 patients.[8]

Although it is considered possible there is no direct evidence of the virus being transmitted by skin to skin contact.[10] A person could get COVID-19 indirectly by touching a contaminated surface or object before touching their own mouth, nose, or eyes,[11] though this is not thought to be the main way the virus spreads, and there is no direct evidence of this method either.[3][10] The virus is not known to spread through feces, urine, breast milk, food, wastewater, drinking water, or via animal disease vectors (although some animals can contract the virus from humans).[11][12] It very rarely transmits from mother to baby during pregnancy.[10] Social distancing and the wearing of cloth face masks, surgical masks, respirators, or other face coverings are controls for droplet transmission. Transmission may be decreased indoors with well maintained heating and ventilation systems to maintain good air circulation and increase the use of outdoor air.[3] It has been suggested that improving ventilation and managing exposure duration can reduce transmission.[5][13]

The number of people generally infected by one infected person varies;[10] as of September 2020 it was estimated that one infected person will, on average, infect between two and three other people.[14] This is more infectious than influenza, but less so than measles.[15] It often spreads in clusters, where infections can be traced back to an index case or geographical location.[16] There is a major role of "super-spreading events", where many people are infected by one person.[10][17][18] A person who is infected can transmit the virus to others up to two days before they themselves show symptoms, and even if symptoms never appear.[3][14] People remain infectious in moderate cases for 7–12 days, and up to two weeks in severe cases.[14] In October 2020, medical scientists reported evidence of reinfection in one patient.[19][20]


Transmission is believed to occur largely through virus being breathed out inside tiny respiratory droplets by an infected person, and then inhaled by a susceptible person. We breathe out droplets with a wide range of sizes and there is debate about the relative importance to transmission of smaller and larger sizes of these droplets.[3][9][21][10][22] Whenever we breathe, cough, sneeze or talk we produce respiratory droplets. These droplets can land in the mouths or noses of people who are nearby, or possibly be inhaled into the lungs. Spread is believed to be more likely when a susceptible person is in close contact with an infected person as there the air will have a higher concentration of these droplets.[2][14][15]

Social distancing and the wearing of cloth face masks, surgical masks, respirators, or other face coverings aim to reduce droplet transmission.[15][14]

Social distancing

The World Health Organization recommends 1 meter (3.3 ft) of social distancing.[2] The European Centre for Disease Prevention and Control states that close proximity is "usually less than 1 meter (3.3 ft) apart."[14] The U.S. Centers for Disease Control and Prevention (CDC) defines close contact as "within 6 feet (1.8 m) of an infected person for a cumulative total of 15 minutes or more over a 24-hour period."[23] The Australian Health Department defines it as sharing an enclosed space for a prolonged period such as two hours.[24][25] However infectious virus has been found by sampling air more than 2 m from an infected person,[26] showing that virus can spread many metres across a room. So social distancing is expected to reduce but not eliminate transmission.


Our breath contains small droplets, at a variable concentration, but typically around 100 per litre of breath. We breath out around 10 litres per minute (more if exercising) of air, so this is about 100 droplets per second breathed out.[27] The vast majority of these droplets are a few micrometres across or smaller, but a few may up to about a millimetre.[27][28] These droplets we breath out are suspended in air, so are by definition aerosols. The largest (100 micrometres and larger) droplets will fall to the ground within a few seconds, but the remainder will remain suspended in air, for minutes or more.[28] The SARS-CoV-2 virus is about 100 nanometres across and is small enough to fit inside all but the very smallest of these droplets. So the virus can leave the body of a infected person, and enters the air, though these droplets in their breath.[28] Then if a susceptible person inhales these droplet-containing virus, they can become infected. Transmission across the air is often called airborne transmission. Airborne transmission of COVID-19 has been shown to occur through aerosols that are able to stay suspended in the air for longer periods.[2][29][26][30][5]

Understanding transmission through the air is complicated both by the very large range of sizes of the droplets we breathe out, and the fact that our breath is travelling at several centimetres per second when it leaves our mouth, and is typically warmer and more humid than the surrounding air.[28][31][32] So, when a person (not wearing a mask) is talking to you from around two metres or less, you are within a cone of slowly moving air formed by their breath,[31] and this air movement will move the aerosol of droplets towards you. The warmth of breath also affects how the droplets can move across a room, particularly if the surrounding air is cold, the breath (with its suspended aerosol) will tend to rise due to convection.

Evidence for airborne transmission

The evidence is that airborne transmission usually occurs indoors,[33] such as in restaurants, hospitals,[34] choir practices,[35] fitness classes, nightclubs, offices, and religious venues, often when they are crowded or less ventilated.[8][5] An example of airborne transmission is believed to be a superspreading event that occurred in a Skagit Valley chorale practice, with an estimated 32 to 52 of the 61 attendees becoming infected.[36][35] An existing model of airborne transmission (the Wells-Riley model) was adapted to help understand why crowded and poorly ventilation spaces promote transmission.[35] Airborne transmission also occurs in healthcare settings, often when aerosol-generating medical procedures performed on COVID-19 patients.[8] Long distance dispersal of virus particles has been detected in ventilation systems of a hospital, indicating the possibility of long range aerosol transmission.[34]

Multiple outbreaks have been reported in indoor locations where infected persons spend long periods of time, such as restaurants and nightclubs.[2][37] This has particularly shown how the virus can spread through the airborne route.[11][1] There are several well documented examples where COVID-19 has transmitted despite there not being the close proximity required for droplet transmission, including choir practice in Washington, a restaurant in Guangzhou,[38] and a tour bus in Hunan.[22] It is also believed that the risk of transmission is lower outside and in rooms with good ventilation.[39] The restaurant in Guangzhou was poorly ventilated[38] (less than one air change per hour, much less than is recommended), and it is also notable that there was no close contact between the infected person, and those who were infected, who remained a few metres away. Thus aerosol not fomite transmission seems likely.[38]


Aerosol transmission of the COVID-19 virus outside of medical facilities has been the subject of controversy, with the WHO initially considering it insignificant, which led to widespread criticism from scientists. In July 2020, the WHO changed its guidance, saying that short-range aerosol transmission cannot be ruled out in these situations.[40] In October 2020, it then further changed its guidance, recognizing that although current evidence suggests respiratory droplets is the main method, airborne transmission is occurring, particularly in high risk indoor settings, where there is crowding and less ventilation. It states to avoid the "Three C"s - crowded places, close contact settings, and confined and enclosed spaces.[8]

The U.S. CDC has also been criticized for delays in informing the public about airborne transmission, with John Allan from Harvard University's School of Public Health writing "many scientists have known that airborne transmission of the virus was happening since February. The CDC somehow failed to recognize the accumulating evidence that airborne transmission is important and therefore failed to alert the public."[41][42]

In Canada, the controversy has been attributed to complexities involving the N-95 mask supply chain, and fears that it may run out.[43] The Public Health Agency of Canada recognised airborne transmission in November 2020, stating the relative importance between airborne transmission and large droplet transmission is unknown.[9]

In Australia the controversy involves PPE guidelines, with the Australian Medical Association accusing Professor Lyn Gilbert, chair of the Infection Control Expert Group, of disparaging frontline healthcare workers, accusing them of poor habits and being apathetic.[44][45][46] She argues that provision of N-95 masks gives clinicians a false sense of security, and they are unable to be trained to use them properly, placing them more at risk. She also claims that washing hands after touching contaminated surfaces is more important than use of a respirator, which is recommended for only extraordinary circumstances.[47]

Other sources

Medical procedures

A tube and a camera being inserted into the mouth of a patient
Tracheal intubation is an example of an aerosol-generating procedure that carries increased risk of transmission from an infected patient.

Some medical procedures performed on COVID-19 patients in health facilities are aerosol-generating,[2][48] i.e., generate aerosols over and above those produced by a patient breathing. WHO recommends the use of filtering facepiece respirators such as N95 masks or FFP2 masks in settings where aerosol-generating procedures are performed,[11] while CDC and the European Centre for Disease Prevention and Control recommend these controls in all situations involving the care of COVID-19 patients (other than during crisis shortages), as respirators are designed to protect the wearer, whereas surgical masks are designed to protect the patient.[49][50][51]

Toilet flushes

There has been concern that toilet aerosols generated by flushing contaminated toilets may spread the COVID-19 virus. WHO recommends that people suspected or confirmed with COVID-19 should use their own toilet, and while flushing the toilet lid should be down to block both droplets and aerosol clouds.[12]

Direct contact

Physical intimacy

The virus spreads through saliva and mucus, and kissing can easily transmit COVID-19. It is possible that direct contact with feces including anilingus may also lead to virus transmission.[52] However, as of July 2020 there have been no published reports of COVID-19 transmission through feces or urine.[11] While COVID‑19 is not a sexually transmitted infection, physical intimacy carries a high risk of transmission due to close proximity.[53]

Hand washing is a control against direct contact transmission.[15] Others include avoiding kissing and avoiding casual sex.[52][53] During physical intimacy, barriers such as face masks, condoms, or dental dams can be used, and socially distanced intimacy can be practiced through mutual masturbation or cybersex.[52]

Mother to child

As of July 2020, there were no cases of transmission from mother to baby during pregnancy.[11][54] Studies have found no viable virus in breast milk.[11] The WHO recommends that mothers with suspected or confirmed COVID-19 should be encouraged to initiate or continue to breastfeed.[11][55]

Indirect contact

Objects and surfaces

A silver-colored door handle on a white door
Surfaces that are often touched such as door handles may transmit COVID-19, although is not thought to be the main way the virus spreads.

It may be possible that 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 possibly their eyes.[56] This is not thought to be the main way the virus spreads.[2][3][10][11][14][15] Viable virus or RNA has been detected on contaminated surfaces for periods ranging from hours to days, depending on environmental conditions. As of July 2020, there were no specific reports which directly demonstrated transmission via fomites, although fomite transmission is hard to distinguish from transmission from the infectious person themselves, and fomite transmission was still considered a likely mode of transmission.[11]

On surfaces the amount of viable active virus decreases over time until it can no longer cause infection. One study indicated that the virus can be detected for up to four hours on copper, up to one day on cardboard, and up to three days on plastic (polypropylene) and stainless steel (AISI 304).[14][57][58] In October 2020, medical researchers concluded the COVID-19 virus can remain on common surfaces for up to 28 days.[59] It has been shown that sunlight inactivaters the virus, so that indirect transmission occurs mostly indoors.[60]

Since the virus is stable on human skin,[60] hand washing and periodic cleaning of surfaces are controls against indirect contact transmission through fomites.[2] Surfaces are easily decontaminated with household disinfectants which destroy the virus outside the human body. Disinfectants or bleach are not a treatment for COVID‑19, and cause health problems when not used properly, such as when used on or inside the human body.[2][61][62]

Food and water

The risk of food-related transmission is low.[63] Less than 1 in 10,000 contacts with contaminated surfaces, including non-food-related surfaces, leads to infection.[64]

While virus RNA has been found in untreated wastewater, as of May 2020 there is little evidence of infectious virus in wastewater or drinking water.[12][65][66]

After COVID-19 outbreak in Beijing in June, food transmission evidence has been disclosed in China early July 2020 by the detection of SARS-CoV-2 on frozen foods, including their packaging materials and storage environments.[67][68] On Oct 17, China CDC reports that they separated out live SARS-CoV-2 virus from packaging of cold chain imported food, during investigation of another outbreak in Qingdao in October.[69] As of October, there is no direct evidence of general public contracting the virus from contaminated food packaging. [70]

Animal vectors

There are a small number of cases of spread from people to pets, including cats and dogs. Other cases include lions and tigers at a New York zoo, and minks on farms in the Netherlands.[71][72] In a laboratory settings, animals shown to be infected include ferrets, cats, golden Syrian hamsters, rhesus macaques, cynomolgus macaques, grivets, common marmosets, and dogs.[71][73][74] By contrast, mice, pigs, chickens, and ducks do not seem to become infected or spread the infection.[71] There is no evidence that insect disease vectors such as mosquitoes or ticks spread COVID-19.[75]

CDC recommends that pet owners limit their pet's interaction with people outside their household. Face coverings are not recommended on pets because covering a pet's face could harm them, and they should not be disinfected with cleaning products not approved for animal use.[76] People sick with COVID-19 should avoid contact with pets and other animals.[73][76]

The risk of COVID-19 spreading from animals to people is considered to be low. Although the virus likely originated in bats, the pandemic is sustained through human-to-human spread.[71][73] Pets do not appear to play a role in spreading COVID-19, but there are reports from infected mink farms indicating transmission to humans is a possibility.[73]

Other aspects

Effect of vaccination

The already vaccinated people can still spread COVID-19, it is more difficult but possible.[77] So vaccinated people must continue doing preventive measures (washing hands with soap, wearing face masks, keeping social distance) to avoid infecting others, especially to vulnerable people.

Reproduction number

It transmits very easily and sustainably, however the number of people generally infected by one infected person varies.[10] Many people do not transmit the virus, but some transmit to many people; therefore the virus is considered to be "overdispersed".[10][78] As of September 2020 it was estimated that, on average, one infected person will infect between two and three other people.[14] This is more infectious than influenza, but less so than measles.[15]

Estimates of the number of people infected by one person with COVID-19, the R0, have varied. The WHO's initial estimates of R0 were 1.4–2.5 (average 1.95); however an early April 2020 review found the median R0 to be 5.7.[79] In November, a systematic review estimated R0 to be approximately 2.87 (95% CI, 2.393.44).[80]

It often spreads in clusters, where infections can be traced back to an index case or geographical location.[16] There is a major role of "super-spreading events", where many people are infected by one person.[10] These generally occur usually indoors, where groups of people remain in poor ventilation for longer periods.[10] It transmits via aerosols particularly in these crowded and confined indoor spaces, which are particularly effective for transmitting the virus, such as restaurants, nightclubs or choirs.[3][8][81] Another important site for transmission is between members of the same household.[10]

Source control may be used to partially inhibit spread.[citation needed]

Duration of transmissibility

People are most infectious when they show symptoms, even if mild or non-specific, but may be infectious for up to two days before symptoms appear. They remain infectious an estimated seven to twelve days in moderate cases and an average of two weeks in severe cases.[2][14] Research has indicated that viral load peaks around the day of symptom onset and declines after, as measured by RNA studies.[11]

The possibility of transmission for completely asymptomatic cases is unclear.[2][14] A systematic review estimated that the proportion of truly asymptomatic cases ranges from 6% to 41%, with some limitations ("asymptomatic" was not very well defined). Another study with a clear definition of asymptomatic infections estimated a proportion of 23%.[11]

Regarding transmission by indirect contact (i.e., touching contaminated surfaces), the rate of transmission decreases steadily with time,[60] which is why it could be useful to let incoming packages sit untouched for a few days before opening them to reduce the risk of contracting the disease.

See also


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