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March 10, 2020 --Although a 14-day quarantine after exposure to the new coronavirus is "well supported" by evidence, some infected people will not show symptoms until after that period, according to authors of a recent analysis published in Annals of Internal Medicine.
Most people who have COVID-19, the disease caused by the new coronavirus, will get symptoms by day 12 of the infection, which is within the 14-day period of quarantine the CDC recommends, the authors wrote.
But about 101 out of 10,000 people could show symptoms after the end of that 14-day monitoring period, they cautioned.
"Our analyses do not preclude that estimate from being higher," said the investigators, led by Stephen A. Lauer, PhD, of Johns Hopkins Bloomberg School of Public Health in Baltimore.
The analysis, based on 181 confirmed cases of COVID-19 documented outside the outbreak epicenter in Wuhan, China, makes "more conservative assumptions" about how long it takes symptoms to appear and the potential for continued exposure, compared to previous studies, the researchers wrote.
The estimated incubation period for the new coronavirus in the 181-patient study was a median of 5.1 days. That is comparable to previous estimates of COVID-19 cases outside Wuhan and consistent with other known human coronavirus diseases, such as severe acute respiratory syndrome (SARS), which had a reported mean incubation period of 5 days, Lauer and colleagues noted.
Symptoms came on within 11.5 days for 97.5% of patients in the study.
In an interview, Aaron Eli Glatt, MD, chair of medicine at Mount Sinai South Nassau in Oceanside, NY, said that in practical terms, the results suggest that most patients with COVID-19 will be identified within 14 days, with an "outside chance" of an infected person leaving quarantine and transmitting the virus for a short period of time before showing symptoms.
"I think the proper message to give those patients [who do not show symptoms upon leaving quarantine] is, 'after 14 days, we're pretty sure you're out of the woods, but should you get any symptoms, immediately re-quarantine yourself and seek medical care,' " he said.
Study co-author Kyra H. Grantz, a doctoral graduate student at the Johns Hopkins Bloomberg School of Public Health, said extending a quarantine beyond 14 days might be considered in the highest-risk scenarios, though the benefits of doing so would have to be weighed against the costs to public health and to the people under quarantine.
"Our estimate of the incubation period definitely supports the 14-day recommendation that the CDC has been using," she said in an interview.
Grantz emphasized that the estimate of 101 out of 10,000 cases having symptoms after day 14 of active monitoring assumes the "most conservative, worst-case scenario" in a population that is fully infected.
The study was supported by the CDC, the National Institute of Allergy and Infectious Diseases, the National Institute of General Medical Sciences, and the Alexander von Humboldt Foundation. Four authors reported disclosures related to those entities, and the remaining five reported no conflicts of interest.
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