Am I safe to visit high-risk relatives if I just had a negative COVID-19 nasal swab?
There are a few components to explore in answering this question.
First, when you test in relation to possible exposure, is important. If you had a possible exposure 3 days prior to your COVID-19 nasal swab, you could still develop COVID-19 over a week after the negative swab – that swab just meant you hadn’t built an infection enough to be shedding virus yet. A study on incubation period published in the Annals of Internal Medicine estimates 50% of people develop symptoms by day 5, 75% by day 7 and >90% by day 11 after exposure.
Second, we know that the “presymptomatic” period is a major source of exposure risk. A study based on data in and around China suggests that 44% of transmission comes from people who are about to get sick, but don’t yet know they are infected. We know the amount of virus builds over time, peaking at or right before symptom onset. COVID-19 tests are better at detecting virus in this time frame. It’s still unclear how good tests are at finding presymptomatic cases before that time, when viral shedding is less but growing.
When considering your risk for exposing others, a negative COVID-19 nasal swab can be part of your assessment, but know that it’s not fail-proof. We recommend weighing your overall risk into the equation, borrowing a rhyme from Bill Miller, an epidemiologist and physician at Ohio State University, uses: Time, space, people and place. If your COVID-19 test is negative in the setting of having spent little time around other people, with a lot of space between you and anyone you did see, with those people being people you know well who were socially distancing carefully themselves and you saw them in an outdoor environment, you are probably pretty low risk.