The modes of spread of COVID‑19, a new disease caused by the SARS-CoV-2 virus, are under research and investigation. It spreads from person to person, via several different modes, mainly when people are in close proximity to one another through respiratory droplets.^[1][2] It's currently estimated that one infected person will on average infect between two and three other people.^[3] This is more infectious than influenza, but less so than measles.^[1][4] It can transmit when people are symptomatic, also for up to two days prior to developing symptoms, and even if a person never shows symptoms.^[3][1] People remain infectious in moderate cases for 7-12 days, and up to two weeks in severe cases.^[3]

The disease seems to spread mainly after an infected person breathes, coughs, sneezes, talks or sings.^[1] Actions such as these produce contaminated droplets, which travel through the air, land on the mouths or noses of others close by, and can be inhaled into the lungs, thereby causing new infection.^[1] Many of these droplets are too heavy to linger in the air and fall to the ground.^[5] However smaller droplets become airborne, suspended in the air for longer periods of time.^[1][6][7] The relative importance of the smaller aerosols (known as droplet nuclei, which cause airborne disease) is unknown.^[1] Airborne transmission occurs particularly in crowded and less ventilated indoor spaces, which are particularly effective for transmitting the virus, such as restaurants, nightclubs, public transport and gatherings such as funerals.^[7][5][1] It also can occur in the healthcare setting, where certain medical procedures performed on COVID-19 patients generate aerosols.

It may be possible that a person can get COVID-19 through indirect contact by touching a contaminated surface or object, and then touching their own mouth, nose, or possibly their eyes,^[2] though this is not thought to be the main way the virus spreads, and it has not been conclusively demonstrated.^[1] Kissing, physical intimacy and other forms of direct contact can easily transmit the virus and thus lead to COVID-19 in people exposed to such contact.

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.^[1]

There currently is no significant evidence of COVID-19 virus transmission through feces, urine, breast milk, food, wastewater, drinking water, animal disease vectors, or from mother to baby during pregnancy, although research is ongoing and caution is advised.^[2][8]

Routes

Respiratory droplets

The COVID-19 virus spreads through respiratory droplets produced when an infected person breathes, coughs, sneezes, or talks. These droplets can land in the mouths or noses of people who are nearby, or possibly be inhaled into the lungs. Spread is more likely when people are in close contact within about 6 feet.^[4][3][5]

Social distancing and the wearing of cloth face masks, surgical masks, respirators, or other face coverings are controls for droplet transmission.^[4][3] The World Health Organization recommends 1 meter (3.3 ft) of social distance;^[5] the U.S. Centers for Disease Control and Prevention recommends 6 feet (1.8 m).^[4]

Aerosols

Airborne transmission of COVID-19 has been shown to occur through aerosols, smaller droplets that are able to stay suspended in the air for longer periods of time.^[5]

Multiple outbreaks have been reported in indoor locations where infected persons spend long periods of time, such as restaurants and nightclubs.^[9][5] The role of short-range aerosol transmission in these cases is unknown. According to the WHO, as of July 2020, while viral RNA had been found in air samples, viable virus had not; there were no cases of transmission to health workers exposed to COVID-19 cases when contact and droplet precautions were appropriately used; and experimental studies such as one that found viable virus in aerosols generated using a high-powered jet nebulizer do not reflect normal human cough conditions.^[2] However, there have been several individual cases where COVID-19 was transmitted despite there not being the close proximity required for droplet transmission and where contact transmission was unlikely. These include a choir practice in Washington, a restaurant in Guangzhou, and a tour bus in Hunan.^[10]

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.^[10]

Medical

Some medical procedures performed on COVID-19 patients in health facilities are aerosol-generating.^[5][11] WHO recommends the use of filtering facepiece respirators such as N95 masks or FFP2 masks in settings where aerosol-generating procedures are performed, while CDC and the European Centre for Disease Prevention and Control recommend these controls in all situations involving the care of COVID-19 patients.^[2]

Toilet

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.^[8]

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.^[12] However, as of July 2020 there have been no published reports of COVID-19 transmission through feces or urine.^[2] While COVID‑19 is not a sexually transmitted infection, physical intimacy carries a high risk of transmission due to close proximity.^[13]

Hand washing is a control against direct contact transmission.^[4] Others include avoiding kissing and avoiding casual sex.^[12][13] 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.^[12]

Mother to child

As of July 2020, there were no cases of transmission from mother to baby during pregnancy.^[2][14] Studies have found no viable virus in breast milk.^[2] The WHO recommends that mothers with suspected or confirmed COVID-19 should be encouraged to initiate or continue to breastfeed.^[15][2]

Indirect contact

Objects and surfaces

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. This is not thought to be the main way the virus spreads.^[4][3][5] 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).^[3][16][17] There is no evidence of transmission through postal mail, coins, or banknotes.^{[citation needed]}

Hand washing and periodic cleaning of surfaces are controls against indirect contact transmission through fomites.^[5] Surfaces are easily decontaminated with household disinfectants which destroy the virus outside the human body. Note that 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.^[18]

Food and water

There is currently no evidence to support transmission of COVID-19 associated with food.^[19][20]

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.^[8][21][22]

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.^[23][24] In a laboratory settings, animals shown to be infected include ferrets, cats, golden Syrian hamsters, rhesus macaques, cynomolgus macaques, grivets, common marmosets, and dogs.^[23][25] By contrast, mice, pigs, chickens, and ducks do not seem to become infected or spread the infection.^[23] There is no evidence that insect disease vectors such as mosquitoes or ticks spread COVID-19.^[19]

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.^[26] People sick with COVID-19 should avoid contact with pets and other animals.^[25][26]

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.^[23][25] 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.^[25]

Reproduction number

COVID-19 spreads easily between people—easier than influenza but not as easily as measles.^[4]

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 and the basic R₀ (without control measures) to be as high as 8.9.^[27]

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.^[3][5] Research has indicated that viral load peaks around the day of symptom onset and declines after, as measured by RNA studies.^[2]

The possibility of transmission for completely asymptomatic cases is unclear.^[3][5] A recent systematic review estimated that the proportion of truly asymptomatic cases ranges from 6% to 41%, with some limitations ("asymptomatic" was not very well defined). A recent study from China with a clear definition of asymptomatic infections estimated a proportion of 23%.^[2]