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.
I have heard a lot about the incubation period for COVID-19, but am confused about how that interacts with testing for active infections (not antibodies). For example, if I am exposed/infected on Monday and tested on Tuesday (likely before showing symptoms), should I expect that the test would come back positive? Or would it take some time to get a positive result due to the incubation period or for other reasons?
While early research on asymptomatic transmission has shown that some asymptomatic individuals will test positive two to three days before developing symptoms, we would definitely not expect a positive diagnostic test for COVID-19 one day after exposure.
The diagnostic test, known as a “PCR test,” works by detecting genetic material from SARS-CoV-2, the virus that causes COVID-19, in the nose and upper throat. A study that examined false-negative rates post-exposure, found that during the four days of infection prior to symptom onset, the probability of a false negative on the PCR test went from 100 percent on Day 1 to 67 percent on Day 4. And even on the day individuals began showing symptoms, the false negative rate was still 38 percent, dropping to 20 percent three days after symptom onset. Of course, much depends on the sensitivity of the particular test being used. MIT Medical is using a test that has been shown to have a false-negative rate of less than 5 percent five days post exposure.
On average, symptoms of the virus develop five to six days post exposure, but the incubation period can be as long as 14 days. And, of course some people never develop noticeable or debilitating symptoms — hence the recommendation to self-quarantine and self-monitor for a full two weeks after any likely exposure.
All of this is why we’re not recommending that people make decisions about their activities or contacts based on the results of a negative test shortly after possible exposure. For example, it’s not a good idea to fly into Boston on a crowded flight, get a COVID-19 diagnostic test within a day or two of arrival, and then, based on a negative result, visit your elderly grandparents. The only thing that negative test can tell you is that, at that particular moment in time, your sample did not show viral levels high enough to be reliably measured. It does not mean you were not exposed and infected during your travels. It does not mean you were not exposed and infected after your arrival. Do you want to visit your grandparents after flying into Boston? Self quarantine for 14 days first.
On the other hand, if you get a positive test, you are almost certainly infected, because the false-positive rate is very low. But because a false negative is possible at any stage of infection, it’s important to get tested at the optimal time, when you are most likely to test positive if you are infected. Based on our current knowledge, that window would be approximately five to seven days after a probable exposure.