The 14-day quarantine period put forward by WHO and the CDC was based on smaller studies of sicker hospitalized patients. But an estimated 80% of adult COVID-19 patients are not sick enough to be hospitalized, and people can be infectious with the new coronavirus well before they develop symptoms. The new study found that the median incubation period was seven days for adults and nine days for children, far longer than the mean of 5.2 days from an earlier study out of Wuhan.

For that study, researchers from the Johns Hopkins Bloomberg School of Public Health and two other universities analyzed 181 cases of COVID-19 in 25 countries, from early January to late February. They found that 97.5% of people who developed symptoms did so within 11.5 days of exposure.

However, the researchers extrapolated that for every 10,000 exposed individuals, 101 would develop symptoms after a 14-day quarantine. Reuters reported on one such case in late February, that of a 70-year-old man in China’s Hubei Province who did not show symptoms until 27 days after he was infected.