Transmission of COVID-19

COVID‑19 is a new disease, and many of the details of its transmission are still under investigation. COVID-19 is thought to spread mainly through respiratory droplets produced when an infected person coughs, sneezes, or talks. Social distancing and the wearing of cloth face masks, surgical masks, respirators, or other face coverings are controls for droplet transmission. 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, though this is not thought to be the main way the virus spreads. Kissing, physical intimacy and other forms of direct contact can easily transmit COVID-19.

Airborne transmission may also occur through aerosols, smaller droplets that are able to stay suspended in the air for longer periods of time. Some medical procedures performed on COVID-19 patients in health facilities are aerosol-generating and are a cause of transmission. The role of short-range aerosol transmission in crowded and inadequately ventilated indoor locations where infected persons spend long periods of time, such as restaurants and nightclubs, is unknown, but cannot be ruled out.

There is no evidence of significant COVID-19 transmission through feces, urine, breast milk, food, wastewater, drinking water, animal disease vectors, or from mother to baby during pregnancy.

Routes

Respiratory droplets
The main route of transmission of COVID-19 is respiratory droplets expelled from the mouth and nose when a person sneezes, coughs, or speaks.

COVID-19 spreads mainly through respiratory droplets produced when an infected person 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.[1][2][3]

Social distancing and the wearing of cloth face masks, surgical masks, respirators, or other face coverings are controls for droplet transmission.[1][2] The World Health Organization recommends 1 meter (3 ft) of social distance;[3] the U.S. Centers for Disease Control and Prevention recommends 6 feet (2 m).[1]

Aerosols

Airborne transmission may also occur through aerosols, smaller droplets that are able to stay suspended in the air for longer periods of time.[3]

Respiratory

Some outbreaks have been reported in crowded and inadequately ventilated indoor locations where infected persons spend long periods of time, such as restaurants and nightclubs.[3] 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 using a high-powered jet nebulizer do not reflect normal human cough conditions.[4] 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.[5]

Aerosol transmission of COVID-19 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.[5]

Medical
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][6] 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.[4]

Toilet

There has been concern that toilet aerosols generated by flushing contaminated toilets may spread COVID-19. 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.[7]

Direct contact

Physical intimacy
Kissing carries high risk of COVID-19 transmission due to high levels of virus in the saliva.

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.[8] However, as of July 2020 there have been no published reports of COVID-19 transmission through feces or urine.[4] While COVID‑19 is not a sexually transmitted infection, physical intimacy carries a high risk of transmission due to close proximity.[9]

Hand washing is a control against direct contact transmission.[1] Others include avoiding kissing and avoiding casual sex.[8][9] 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.[8]

Mother to child

As of July 2020, there were no cases of transmission from mother to baby during pregnancy.[2][4] Studies have found no viable virus in breast milk.[4]

Indirect contact

Objects and surfaces
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. This is not thought to be the main way the virus spreads.[1][2][3] On surfaces the amount of viable active virus decreases over time until it can no longer cause infection. Although one study indicated that SARS-Cov-2 remains viable on cardboard, copper, and stainless steel for varying amounts of time up to three days, there is no evidence of transmission through postal mail, coins, or banknotes.[2]

Hand washing and periodic cleaning of surfaces are controls against indirect contract transmission through fomites.[3] 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.[10]

Food and water

There is currently no evidence to support transmission of COVID-19 associated with food.[2][11]

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.[7][12]

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 and a New York zoo, and minks on farms in the Netherlands.[13][14] In a laboratory settings, animals shown to be infected include ferrets, cats, golden Syrian hamsters, rhesus macaques, cynomolgus macaques, grivets, common marmosets, and dogs.[13][15] By contrast, mice, pigs, chickens, and ducks do not seem to become infected or spread the infection.[13] There is no evidence that insect disease vectors such as mosquitoes or ticks spread COVID-19.[11]

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.[16] People sick with COVID-19 should avoid contact with pets and other animals.[15][16]

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.[13][15] 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.[15]

Reproduction number

COVID-19 spreads easily between people—easier than influenza but not as easily as measles.[1]

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 basic R0 (without control measures) to be higher at 3.28 and the median R0 to be 2.79.[17]

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][3] Research has indicated that viral load peaks around the day of symptom onset and declines after, as measured by RNA studies.[4]

The possibility of transmission for completely asymptomatic cases is unclear.[2][3] A June 2020 review found that 40–45% of infected people are asymptomatic.[18]

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References
  1. "How COVID-19 Spreads". U.S. Centers for Disease Control and Prevention. 2020-06-16. Retrieved 2020-07-24.
  2. "Q & A on COVID-19". European Centre for Disease Prevention and Control. 2020-04-24. Retrieved 2020-08-13.
  3. "Q&A: How is COVID-19 transmitted?". World Health Organization. 2020-07-09. Retrieved 2020-08-13.
  4. "Transmission of SARS-CoV-2: implications for infection prevention precautions". World Health Organization. 2020-07-09. Retrieved 2020-08-13.
  5. Lewis, Dyani (2020-07-23). "Mounting evidence suggests coronavirus is airborne — but health advice has not caught up". Nature. doi:10.1038/d41586-020-02058-1. Retrieved 2020-08-13.
  6. Tran K, Cimon K, Severn M, Pessoa-Silva CL, Conly J (2012). "Aerosol generating procedures and risk of transmission of acute respiratory infections to healthcare workers: a systematic review". PLOS ONE. 7 (4): e35797. Bibcode:2012PLoSO...735797T. doi:10.1371/journal.pone.0035797. PMC 3338532. PMID 22563403.
  7. "Water, sanitation, hygiene, and waste management for SARS-CoV-2, the virus that causes COVID-19". World Health Organization. 2020-07-29. Retrieved 2020-08-13.
  8. "Sex and Coronavirus Disease 2019 (COVID-19)" (PDF). New York City Department of Health. 2020-08-06. Retrieved 2020-08-13.
  9. "COVID-19 and Our Communities". ACON (New South Wales). 2020-08-03. At section "Sex and COVID-19". Retrieved 2020-08-13.
  10. "COVID-19: Cleaning And Disinfecting Your Home". U.S. Centers for Disease Control and Prevention. 2020-05-27. Retrieved 2020-08-13.
  11. "Coronavirus Disease 2019 (COVID-19): Frequently Asked Questions". U.S. Centers for Disease Control and Prevention. 2020-07-15. Retrieved 2020-07-24.
  12. "Information for Sanitation and Wastewater Workers on COVID-19". U.S. Centers for Disease Control and Prevention. 2020-05-28. Retrieved 2020-08-13.
  13. "COVID-19 and Animals". U.S. Centers for Disease Control and Prevention. 2020-06-22. Retrieved 2020-07-24.
  14. "USDA Statement on the Confirmation of COVID-19 in a Tiger in New York". United States Department of Agriculture. 5 April 2020. Archived from the original on 15 April 2020. Retrieved 16 April 2020.
  15. "Questions and Answers on COVID-19". World Organisation for Animal Health. 2020-06-09. Retrieved 2020-07-23.
  16. "COVID-19: If You Have Pets". U.S. Centers for Disease Control and Prevention. 2020-06-28. Retrieved 2020-07-24.
  17. "Novel Coronavirus: Information for Clinicians" (PDF). Australian Government Department of Health. 2020-04-03. Retrieved 2020-08-13.
  18. Oran, DP; Topol, EJ (3 June 2020). "Prevalence of Asymptomatic SARS-CoV-2 Infection: A Narrative Review". Annals of Internal Medicine. doi:10.7326/M20-3012. PMC 7281624. PMID 32491919.
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