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Transmission of COVID-19

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Transmission of COVID-19
Other namesMode of spread of COVID-19
Covid-19 Aerosol.jpg
The respiratory route of spread of COVID-19, encompassing larger droplets and aerosols.
SpecialtyInfection prevention and control
TypesRespiratory droplet, airborne transmission, fomites
PreventionFace coverings, quarantine, physical/social distancing, ventilation, hand washing, vaccination


The transmission of COVID-19 is the passing of coronavirus disease 2019 from person to person. The disease is mainly transmitted via the airborne route when people inhale droplets and small airborne particles (that form an aerosol) that infected people breath out as they breathe, talk, cough, sneeze, or sing.[1][2][3] Infected people are more likely to transmit COVID-19 when they are physically close. However, infection can occur over longer distances, particularly indoors.[1][4]

Infectivity begins as early as three days before symptoms appear, and people are most infectious just prior to and during the onset of symptoms.[5] It declines after the first week, but infected people remain contagious for up to 20 days. People can spread the disease even if they are asymptomatic.[6][7][5]

Infectious particles range in size from aerosols that remain suspended in the air for long periods of time to larger droplets that remain airborne or fall to the ground.[8][9] Various groups utilise terms such as "airborne" and "droplet" both in technical and general ways, leading to confusion around terminology.[10] Additionally, COVID-19 research has redefined the traditional understanding of how respiratory viruses are transmitted.[9][11] The largest droplets of respiratory fluid do not travel far, and can be inhaled or land on mucous membranes on the eyes, nose, or mouth to infect.[8] Aerosols are highest in concentration when people are in close proximity, which leads to easier viral transmission when people are physically close,[8][9][11] but airborne transmission can occur at longer distances, mainly in locations that are poorly ventilated;[8] in those conditions small particles can remain suspended in the air for minutes to hours.[8]

The number of people generally infected by one infected person varies;[12] as only 10 to 20% of people are responsible for the disease's spread.[13] It often spreads in clusters, where infections can be traced back to an index case or geographical location.[14] Often in these instances, superspreading events occur, where many people are infected by one person.[12]

A person can get COVID-19 indirectly by touching a contaminated surface or object before touching their own mouth, nose, or eyes,[5][15] though strong evidence suggests this does not contribute substantially to new infections.[8] Although it is considered possible, there is no direct evidence of the virus being transmitted by skin to skin contact.[12] The virus is not known to spread through feces, urine, breast milk, food, wastewater, drinking water, or animal disease vectors (though some animals can contract the virus from humans).[15][16] It very rarely transmits from mother to baby during pregnancy.[12]

Infectious period

After people are infected with COVID-19, they are able to transmit the disease to other people from one to three days before developing symptoms, known as presymptomatic transmission.[5] Contact tracing is used to find and contact people whom have been in contact with an infected individual in the 48 to 72 hours before they develop symptoms, or before their test date if asymptomatic.[5]

People are most infectious when they show symptoms—even if mild or non-specific—as the viral load is highest at this time.[5][15] They remain infectious, on average, seven to twelve days in moderate cases, and two weeks in severe cases.[17][18][15]

Asymptomatic transmission

People who are completely asymptomatic are able to transmit the virus.[8] A systematic review estimated that the proportion of asymptomatic cases ranges from 6 to 41%.[15] Although asymptomatic, they can have the same viral load as symptomatic and presymptomatic cases, and are able to transmit the virus.[5] However, the infectious period of asymptomatic cases has been observed to be shorter with faster viral clearance.[5]

Modes of transmission

Our breath, shown here when speaking, forms a roughly cone-shaped plume of warm humid air, that breaks up into rolls.[19] 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).

An infected person breathes out the virus in small liquid droplets. Our breath forms a roughly cone-shaped plume of air, that carries out the virus-containing droplets.[20][19] So, we expect the risk of transmission is higher when a susceptible person is in close proximity to an infected person, especially when they face the infected person. However, transmission can occur across a room.[4][21] The risk of transmission from all size droplets and aerosols is always lower with good indoor ventilation or outdoors.[22]

Small droplets suspended in air are, by definition, an aerosol, but multiple terms have been used to describe particles/droplets of different sizes, and some are used specifically and technically by some groups, and generally by others. This introduces confusion in describing how COVID-19 transmits.[23]

Airborne or aerosol transmission

Although we lack definitive evidence, it is likely that the dominant mode of transmission is direct airborne or aerosol transmission.[24][25][26] This is where a droplet with virus is breathed out by an infected person, enters the surrounding air, and then is breathed in by another person. All of us (infected or not) continuously breathe out droplets with sizes ranging from less than a micrometre to over 100 micrometres from exhalation.[27] These droplets contain the virus for (some) infected people.[28][29]

The largest droplets produced during exhalation are generally more than 100 micrometres in diameter, and may fall to the ground within seconds due to gravity.[30][31] These droplets, referred to as "respiratory fluids" that are "splashed or sprayed" by the US CDC, can only transmit COVID-19 at short distances—a metre or a few metres.[8]

Video explainer on reducing airborne transmission of COVID-19 indoors

Droplets of smaller than 100 micrometres in size can stay suspended in the air for at least minutes and move across a room.[17][32][33][20] 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 less well ventilated spaces. This airborne transmission usually occurs indoors in high risk locations,[34] including restaurants, hospitals,[35] choir practices,[4] fitness classes, nightclubs, offices, and religious venues.[36][25] In healthcare settings, guidelines have noted that behaviours such as coughing contribute to aerosol transmission, with appropriate airborne precautions needed.[37] The half-life of SARS-CoV-2 in aerosols is approximately 1.1 to 1.2 hours.[38]

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.[39] 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,[39] 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 filters to remove the virus-containing particles.

Examples

Repeatedly documented examples of airborne transmission have been shown, generally where infected persons spend long periods of time, such as restaurants and nightclubs.[17][40][41] They occur despite there not being the close proximity required for the previously theorised droplet transmission. Early examples included a choir practice in Washington, a restaurant in Guangzhou,[21] a tour bus in Hunan,[42] and a church in Sydney.[43] The restaurant was poorly ventilated[21] (less than one air change per hour, much less than is recommended), and there was no close contact between the infected person and those who were infected, the latter of whom remained a few metres away.[21]

A superspreading event in a Skagit County, Washington, choral practice resulted in 32 to 52 of the 61 attendees infected.[44][4] An existing model of airborne transmission (the Wells-Riley model) was adapted to help understand why crowded and poorly ventilation spaces promote transmission.[4] Airborne transmission also occurs in healthcare settings; long distance dispersal of virus particles has been detected in ventilation systems of a hospital.[35]

Controversy

Aerosol transmission of SARS-CoV-2 has been the subject of controversy. The WHO initially considered it to be 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.[45] In October 2020, it recognised that while evidence suggested the main mode of transmission was by respiratory droplets, airborne transmission is occurring, particularly in high risk indoor settings, where there is crowding and less ventilation. The advice was further changed in April 2021 to plainly state: "A person can be infected when aerosols or droplets containing the virus are inhaled." It advises avoiding the "Three C's": crowded places, close contact settings, and confined and enclosed spaces.[36]

The US CDC has been criticised 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."[46][47] Since then the CDC has updated its guidance to reflect the importance of airborne/aerosol transmission.[48]

In Canada, the controversy has been attributed to complexities involving the N-95 respirator supply chain and fears that it may run out.[49] 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.[50]

In Australia, the controversy involved PPE guidelines, with varying expert bodies disagreeing on how to best protect healthcare workers.[51][52][53] In 2021, a new guideline was created by a new body, which endorsed the use of N-95 respirators in most circumstances.[54]

Aerosol-generating medical procedures

An infected person breathes out the virus when they breathe or speak.[55] An aerosol of virus-containing droplets suspended in air, is produced in the lungs, throat and mouth, simply by the motion of breathing.[20] However, 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 procedures[17][56] (AGP). This has been done without measuring the aerosols these procedures produce.[57] The aerosols generated by some AGPs have been measured and found to be less than the aerosols produced by breathing[58][59] For example, Continuous positive airway pressure (CPAP) has been designated an AGP but patients provided with CPAP generated less aerosols than those breathing normally.[59] This has led to calls to reconsider AGPs.[57]

The WHO recommends the use of filtering facepiece respirators such as N95 or FFP2 masks in settings where aerosol-generating procedures are performed,[15] 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).[60][61][62]

Other modes of transmission

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.

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,[63] but it is not the main mode of transmission.[17][64][12][15][18][65] 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 have not been specific reports that directly demonstrated transmission via fomites, although fomite transmission is hard to distinguish from transmission from the infectious person themselves.[15]

The amount of viable active virus on surfaces decreases over time until it can no longer cause infection. One study indicated that the virus can be, at a maximum, detected for four hours on copper, one day on cardboard, and three days on plastic (polypropylene) and stainless steel (AISI 304).[18][66][38] In October 2020, medical researchers concluded SARS-CoV-2 can remain on common surfaces for up to 28 days.[67] It has been shown that indirect transmission occurs mostly indoors, as sunlight inactivates the virus.[68]

Since the virus is stable on human skin,[68] hand washing and periodic surface cleaning impede indirect contact transmission through fomites.[17] Surfaces are easily decontaminated with household disinfectants that 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.[17][69][70]

The rate of fomite transmission decreases steadily with time,[68] which is why it can be useful to let incoming packages sit untouched for a few days before opening them to reduce the risk of contracting the disease.

Physical intimacy

A close-up photograph of two people kissing
Kissing carries high risk of COVID-19 transmission due to high levels of virus in saliva.

The virus can spread 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,[71] but as of July 2020 there have been no published reports of COVID-19 transmission through feces or urine.[15] While COVID‑19 is not a sexually transmitted infection, physical intimacy carries a high risk of transmission due to close proximity.[72]

Hand washing impedes direct contact transmission.[65] Other behaviours include the avoidance of kissing and casual sex.[71][72] 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.[71]

Mother to child

As of July 2020, there have been no cases of transmission from mother to baby during pregnancy.[15][73] Studies have not found any viable virus in breast milk.[15] The WHO recommends that mothers with suspected or confirmed COVID-19 should be encouraged to initiate or continue to breastfeed.[15][74]

Food and water

The risk of food-related transmission is low.[75] Less than 1 in 10,000 contacts with contaminated surfaces, including non-food-related surfaces, lead to infection.[76]

While viral RNA has been found in untreated wastewater, as of May 2020 there has been little evidence of infectious virus in wastewater or drinking water.[16][77][78]

After the COVID-19 outbreak in Beijing in June, evidence of food transmission was reported in China in early July 2020 by the discovery of SARS-CoV-2 on frozen foods, packaging materials, and storage environments.[79][better source needed][80] On 17 October, the Chinese CDC reported that they separated out live SARS-CoV-2 from packaging cold chain imported food during an investigation of another outbreak in Qingdao in October.[81] As of October 2020, there has been no direct evidence of general public contracting the virus from contaminated food packaging.[82]

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 (see also Cluster 5 article).[83][84] In laboratory settings, animals shown to be infected include ferrets, cats, golden Syrian hamsters, rhesus macaques, cynomolgus macaques, grivets, common marmosets, and dogs.[83][85][86] By contrast, mice, pigs, chickens, and ducks do not seem to become infected or spread the infection.[83] There is no evidence that insect disease vectors such as mosquitoes or ticks spread COVID-19.[87]

The U.S. CDC recommended that pet owners limit their pet's interactions with people outside their household. Face coverings are not recommended on pets, as covering their faces could harm them, and they should not be disinfected with cleaning products not approved for animal use.[88] People sick with COVID-19 should avoid contact with pets and other animals.[85][88]

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.[83][85] Pets do not appear to play a role in spreading COVID-19, but there are reports from infected mink farms that indicate transmission to humans is a possibility.[85]

Clusters and other patterns

Many people do not transmit the virus, but some transmit to many people, and the virus is considered to be "overdispersed" - the transmission rate has high heterogeneity.[12][89] "Super-spreading events" occur from this minority of infected people, usually in high risk venues including restaurants, nightclubs, places of worship, and they generally occur indoors.[12] It often spreads in these clusters, where infections can be traced back to an index case or geographical location.[14] These generally occur usually indoors, where groups of people remain in poor ventilation for longer periods.[12] 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.[64][36][90] Another important site for transmission is between members of the same household.[12]

As of September 2020 it was estimated that, on average, one infected person can infect two or three other people.[18] This is more infectious than influenza, but less so than measles.[65]

Estimates of the number of people infected by one person with COVID-19, the R0, have varied. In November 2020, a systematic review estimated R0 of the original Wuhan strain to be approximately 2.87 (95% CI, 2.393.44).[91] As of August 2021, the dominant variant is Delta, which has an R0 estimated at 5-9.

Effect of vaccination

People that have been vaccinated can still spread COVID-19, though at reduced rates when compared to someone who has not.[92]

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