You Could Have COVID for This Many Days and Not Know It, Study Says

Previous studies got the virus's incubation period wrong. This study sets the record straight.

Part of what makes coronavirus so highly contagious is its sneaky, asymptomatic incubation period. Until recently, the rule of thumb was that most cases became apparent between one and 14 days after exposure, with most cases revealing themselves with symptoms sometime around the fourth or fifth day. But one new study published in the journal Science Advances has determined patients may remain asymptomatic for longer than previously believed. As it turns out, patients are most likely to show their first symptoms approaching day eight.

While just 10 percent of the infected study subjects remained asymptomatic for a full 14 days, the researchers found that the median incubation period was 7.75 days, three to four days later than most previous estimates.

Of course, understanding COVID's incubation period can make a tremendous difference in whether we successfully contain the virus. "By providing health authorities with a potentially more accurate figure for the incubation period, the results could inform guidelines for containment efforts such as quarantines and studies investigating the disease's transmission," the researchers shared in a press release.

The new study is likely more accurate than previous studies on the subject of incubation because it used the largest patient sample in any such analysis to date. As the researchers pointed out, "The few existing estimates of four to five days were based on small sample sizes, limited data, and self-reports that could be biased by the memory or judgement of the patient or interviewer." This study instead relied on public databases that logged the dates of infection as they occurred, to try to eliminate recall bias.

Finally, the researchers came to one more startling conclusion: that it is possible we've underestimated the total 14-day incubation period. The team determined that there is between a five and 10 percent chance that this is the case—meaning sick individuals may be putting others at risk by ending their quarantines too soon. That's why, as the particulars of the virus continue to unfold, it's wise to err on the side of caution: if you suspect having been exposed, don't wait for symptoms to confirm. When or whether they come doesn't affect your ability to spread coronavirus. And for more on asymptomatic cases, find out why 80 Percent of People in This Age Group Are Asymptomatic.

Lauren Gray
Lauren Gray is a New York-based writer, editor, and consultant. Read more