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I’ve been exposed to COVID-19; how soon will I be 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.

My sister just found out that she was exposed to COVID-19 on Sunday evening. On Monday, she had dinner with my parents and one of my sisters. What are the chances that she could have spread it to them, having just been exposed herself?

Illustration of a rocket with the fuse lit labeled ‘Acme Rocket Co.’ on a desert cliff

It’s difficult to pinpoint exactly when, after exposure to COVID-19, an infected individual would become contagious. However, based on what we know about the incubation period for this virus, there’s almost no chance that your sister could have passed on the virus to your family members just 24 hours after being exposed herself.

While the Centers for Disease Control and Prevention (CDC) and other public-health authorities put the incubation period for the virus at 2–14 days, most people who become ill develop symptoms between five and six days after exposure. This is about the same amount of time needed, on average, for a PCR diagnostic test to be more likely than not to return a true-positive result.

On the other hand, research suggests that people who are infected with SARS-CoV-2, the virus that causes COVID-19, are at their most contagious in the 24 to 48 hours before they experience symptoms. But you would still expect there to be a few days between the time a person is exposed and infected and the time they begin actively shedding virus. If we figure that infected people who become sick typically start experiencing symptoms a bit more than five days after exposure, we can calculate that infectiousness would, on average, begin rising sharply about three days post exposure. In other words, it’s safe to say that it would be exceedingly rare for anyone to transmit the virus earlier than two days post exposure; however, at some point after that, the risk would begin to rise significantly.

So, while your parents should monitor themselves for possible symptoms and continue practicing normal precautions, they shouldn’t worry much, if at all, and there’s no need for them to quarantine for any period of time.

Your sister, on the other hand, isn’t that lucky. Since she knows she was exposed to the virus on Sunday evening, she should follow local regulations for self-quarantine and testing to make sure that she does not infect anyone else if she is, in fact, infected. Regardless of the length of her required quarantine or her test results, it’s important to remember that it can take up to two weeks for some people who are infected to test positive and/or develop symptoms. She should continue to monitor herself closely for symptoms during that time. If your sister does develop symptoms during those 14 days, she will need to remain in isolation for another 10 days from the date her first symptom(s) appeared or until she’s been without a fever for at least 24 hours, whichever is longer.

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