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How long before symptom onset is a person contagious?

July 21, 2021: Since this article was originally published, new variants of the SARS-CoV-2 virus have become prevalent. Vaccination continues to be highly protective against serious illness, hospitalization, and death, even with the more transmissible Delta variant. But we’ve changed some of our recommendations about masking and other precautions for vaccinated individuals in certain situations, even when precautions are not required. Read more.


MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer. 

How long before COVID-19 symptoms appear is a person contagious? In other words, based on the day a person actually becomes ill, how far back should contact tracing go?

Line graph showing levels for how sick you feel and how contagious you are over 4 days with a row of illustrations of people in various social situations, the last being in a medical office

While the incubation period for the virus can be as long as 14 days, research suggests that people who are infected with SARS-CoV-2, the virus that causes the COVID-19 illness, may become infectious to others several days before they start to feel ill. 

A study of 94 patients in China showed that viral load peaked shortly after the onset of symptoms, indicating that people may actually be more infectious in the days before they become ill and before the immune system has a chance to kick in. Another study, which looked at 77 pairs of individuals in which one person infected the other, found that contagiousness both began and peaked before the first symptoms of illness — 2.3 days and 0.7 days respectively. Those researchers concluded that about 44 percent of COVID-19 infections spread from person to person before symptom onset.*

For this reason, the Centers for Disease Control and Prevention (CDC) considers contact tracing to be crucial in protecting communities from further spread of the virus. “Time is of the essence,” they state. “If communities are unable to effectively isolate patients and ensure contacts can separate themselves from others, rapid community spread of COVID-19 is likely to increase to the point that strict mitigation strategies will again be needed to contain the virus.” 

The CDC’s contract-tracing protocol involves identifying and contacting close contacts of individuals who are diagnosed with COVID-19. The CDC identifies a “close contact” as “someone who was within 6 feet of an infected person for at least 15 minutes starting from 48 hours before illness onset until the time the patient is isolated.” Anyone who has been informed that they have had that level of contact with someone who has been positively diagnosed with COVID-19 should stay home, maintain social distancing, and self-monitor until 14 days from the last date of exposure to the infected individual.

*After discovering a syntactical error in their original code, the authors of this study reanalyzed their data and published a correction in August 2020. Though this reanalysis did not change their estimate of the proportion of presymptomatic transmission (44%), it did show that infectiousness peaked at symptom onset rather than slightly earlier, as originally observed. They also found that infectiousness started to rise, on average, earlier than noted in their original paper — at about three days prior to symptom onset, with approximately 9% of viral transmission occurring even earlier. (13 October 2020)

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