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COVID-19: Some Timelines
Let’s say Kwame gets exposed to the SARS-CoV-2 virus on the first day of a month. When would be the best day after that for Kwame to get a test?
Kucirka et al. (Ann Intern Med. 2020 May 13: M20–1495) did a pooled analysis of 7 studies involving 1330 patients that had looked at this question. They found that the best day is Day 8 after exposure. The false-negative rate was 67% on Day 4, fell to 38% on Day 5, and was 20% on Day 8. It started to increase after that.
Now, remember, symptoms appear 5–14 days after exposure to the virus. This means it might be better to wait till symptoms appear before one got tested. The problems with that are twofold. The symptoms might be so mild that one may not notice them — paucisymptomatic. However, the ability to shed the virus and infect others is there. Also, a report from Singapore shows that (Wei et al. MMWR April 10, 2020, / 69(14);411–415), infected persons can shed the virus 1–3 days before the onset of symptoms. Xie He and his group (Nature Medicine 26, 672–675(2020) also showed that the peak of infectiousness may occur during this early period of the infection when symptoms have not yet been set in. Work from a group in Canada showed that this period of peak infectiousness might last 3–5 days after the onset of symptoms. This means before one gets the test on say, Day 8, one could have started shedding the virus already on Day 5…
