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

This article was published on
July 6, 2020

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September 21, 2021: People with asymptomatic cases of COVID-19 can transmit the COVID-19 virus to fully vaccinated individuals. However, the chance of infection is relatively low because symptom-free people are less infectious than people with symptoms, and vaccinated people are much less likely to get infected with COVID-19 than unvaccinated people.

The U.S. Centers for Disease Control and Prevention report from March 19, 2021 estimated that people without symptoms are 75% as infectious as those with symptoms, and people who are vaccinated are estimated to have between 65 - 95% protection against being infected with COVID-19, depending on factors such as underlying medical conditions, the coronavirus variant they are exposed to, and the viral load they are exposed to.

A symptomatic person with COVID-19 would be more likely to transmit the virus to a vaccinated person, and far more likely to transmit the virus to an unvaccinated person.

People who are infected with COVID-19 but are asymptomatic (or symptom free) can spread the virus to others. Researchers estimate that 59% of COVID-19 spread is from people who do not have symptoms. 35% of the spread happens before someone develops symptoms (when they are pre-symptomatic). 24% of COVID-19 spread happens via people who never develop symptoms.

People who are infected with COVID-19 but are asymptomatic (or symptom free) can spread the virus to others. Researchers estimate that 59% of COVID-19 spread is from people who do not have symptoms. 35% of the spread happens before someone develops symptoms (when they are pre-symptomatic). 24% of COVID-19 spread happens via people who never develop symptoms.

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What our experts say

People with asymptomatic (or symptom-free) COVID-19 are infected with the virus, but do not show any symptoms. Common symptoms include cough, fever, or smell or taste changes. Studies have shown that symptom-free people have about the same amount of virus as people with symptoms. This finding suggests that symptom-free people are indeed able to spread the virus.   A U.S. Centers for Disease Control and Prevention report from March 19, 2021 provided new estimates for asymptomatic infection. The report suggested the following:

  • 30% of people infected with COVID-19 stay symptom-free. The previous CDC report from September 2020 suggested that 40% of people infected with COVID-19 stayed asymptomatic.
  • Symptom-free people are less infectious than people with symptoms. Infectiousness is measured by how well someone without symptoms can spread the virus, compared to someone with symptoms. This does not measure how much virus a person is carrying. This report estimated that people without symptoms are 75% as infectious as those with symptoms.
  • 50% of the spread of the COVID-19 happens before people have symptoms.   In a study published in January 2021, researchers estimated that 59% of COVID-19 spread is from people without symptoms. They reported that 35% of the spread happens before someone develops symptoms, which is also called "pre-symptomatic." About a quarter, 24%, happens through people who never develop symptoms. The World Health Organization (WHO) has not provided official estimates of symptom-free spread. WHO experts have noted that estimates will vary across populations.   Because symptom-free people may not know that they are sick, they may not take steps to prevent the spread of the virus. If they are out in the community or spending time close to other people, it is possible that they can spread the virus even more than people who know they are sick and choose to stay home.   People should continue to follow local guidelines to prevent the spread of COVID-19.  Measures like wearing face masks, hand-washing for 20 seconds, and maintaining a distance of 6 feet (2 meters) from others are recommended.

Many COVID-19 vaccines lower the risk of symptomatic and asymptomatic infection. COVID-19 vaccines are recommended for most people. Individuals should discuss COVID-19 vaccines with their doctor or healthcare provider.

Context and background

Experts and healthcare providers worry about the spread of COVID-19 by people without symptoms of illness. People should self-isolate when they are sick. However, without symptoms, not everyone will know if or when they are infected. Symptom-free people 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 (JAMMI)
  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)
  13. Analysis of Asymptomatic and Presymptomatic Transmission in SARS-CoV-2 Outbreak, Germany, 2020 (U.S. CDC)
  14. Occurrence and transmission potential of asymptomatic and presymptomatic SARS-CoV-2 infections: A living systematic review and meta-analysis (Plos Medicine)
  15. SARS-CoV-2 Transmission From People Without COVID-19 Symptoms (JAMA Netw Open)
  16. Impact of the COVID-19 Vaccine on Asymptomatic Infection Among Patients Undergoing Pre-Procedural COVID-19 Molecular Screening (Clinical Infectious Diseases)
  17. Q&A: How is COVID-19 transmitted? (WHO)
  18. COVID-19 Pandemic Planning Scenarios (U.S. CDC)

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