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What do we know so far about the ability of asymptomatic people to transmit the virus?

Update

Asymptomatic people are those that do not show any symptoms but have been infected with COVID-19. As per a recent study published in the medical journal, JAMA, asymptomatic people have similar amounts of virus as symptomatic people and are capable of spreading the virus as much. Because asymptomatic people may not know that they are infected, they may not isolate themselves and that way can spread the virus even more. The U.S. Centers for Disease Control and Prevention (U.S. CDC) has revised its current best estimate and now advises that it estimates 40% of people infected with COVID-19 are asymptomatic. The World Health Organization has yet to provide an official estimate and noted that it will vary across populations. Some studies have noted that asymptomatic carriers might be the biggest spreaders of COVID-19 in certain populations. There is also evidence to suggest that pre-symptomatic people infected with COVID-19 - people who eventually develop symptoms but haven't had any yet - spread the most amount of virus in the time before they have symptoms. This is why it is very important that everyone wear masks (the U.S. CDC recommends wearing a cloth mask over a surgical mask for increased protection, wash their hands vigorously for 20 seconds, and maintain a distance of six feet (2 meters) between themselves and others.

This article was published on
February 17, 2021

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What our experts say

Asymptomatic people are those that do not show any symptoms but have been infected with COVID-19. As per a recent study published in the medical journal, JAMA, asymptomatic people have similar amounts of virus as symptomatic people and are capable of spreading the virus as much. Because asymptomatic people may not know that they are infected, they may not isolate themselves and that way can spread the virus even more.

The U.S. Centers for Disease Control and Prevention (U.S. CDC) has revised its current best estimate and now advises that it estimates 40% of people infected with COVID-19 are asymptomatic. The World Health Organization has yet to provide an official estimate and noted that it will vary across populations.

Some studies have noted that asymptomatic carriers might be the biggest spreaders of COVID-19 in certain populations. There is also evidence to suggest that pre-symptomatic people infected with COVID-19 - people who eventually develop symptoms but haven't had any yet - spread the most amount of virus in the time before they have symptoms.

This is why it is very important that everyone wear masks (the U.S. CDC recommends wearing a cloth mask over a surgical mask for increased protection, wash their hands vigorously for 20 seconds, and maintain a distance of six feet (2 meters) between themselves and others.

Context and background

There is concern about transmission by people without symptoms because this makes disease prevention more challenging. For example, while it is a helpful public health measure to recommend that people self-isolate when ill, it is insufficient to stop the spread of the novel coronavirus. This is because if someone never develops symptoms, or who has not yet developed symptoms, they can still infect others.

Resources

  1. Follow-up of asymptomatic patients with SARS-CoV-2 infection (Clinical Microbiology and Infection)
  2. Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020. (Eurosurveillance)
  3. COVID-19: in the footsteps of Ernest Shackleton (BMJ)
  4. SARS-CoV-2 Community Transmission During Shelter-in-Place in San Francisco (medRxiv)
  5. Estimating the extent of asymptomatic COVID-19 and its potential for community transmission: systematic review and meta-analysis (medRxiv)
  6. Prevalence of Asymptomatic SARS-CoV-2 Infection (Annals of Internal Medicine)
  7. In the W.H.O.’s Coronavirus Stumbles, Some Scientists See a Pattern (NYT)
  8. White House Adviser Warned of Risks of Pandemic; Trump Misleads on Testing (New York Times)
  9. COVID-19 Pandemic Planning Scenarios (U.S. CDC)
  10. Clinical Course and Molecular Viral Shedding Among Asymptomatic and Symptomatic Patients With SARS-CoV-2 Infection in a Community Treatment Center in the Republic of Korea (JAMA)
  11. Can asymptomatic patients spread coronavirus? Here's what a new study reveals (Advisory.com)
  12. Maximizing Fit for Cloth and Medical Procedure Masks to Improve Performance and Reduce SARS-CoV-2 Transmission and Exposure, 2021 (U.S. CDC)

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