Two-week quarantines may not be long enough to identify all people who are sick with COVID-19, according to study published Friday in the journal Science Advances.

Currently, many countries including China, the U.S., and the U.K. impose a 14-day quarantine rule on anyone who may have been exposed to the coronavirus, such as people traveling from areas with high concentrations of COVID-19 cases. But a team led by researchers from Peking University found that up to 10% of infected people could have an incubation period — the time between exposure to the virus and developing symptoms — of over 14 days.

“Policymakers may consider tweaking the 14-day quarantine rule, particularly when there’s a large number of cases,” Zhou Xiaohua, a co-author of the paper, told Sixth Tone. “During an outbreak, even 5% means a lot of infected people.”

Zhou and his team studied 1,084 COVID-19 patients who left the central Chinese city of Wuhan, where cases of the disease were first reported, between Jan. 19 and Jan. 23, when the city was put under lockdown. None of the patients had shown symptoms before leaving Wuhan.

They found that patients tend to carry the virus without showing symptoms for seven to eight days. Previous studies — most involving small samples — had estimated the virus’ incubation period to be four to five days.

Moreover, the team found that 5-10% of cases might not show symptoms for over 14 days. “We may need to extend the quarantine period for people at high risk (of exposure),” Zhou said.

Shen Yinzhong, an infectious disease expert at the Shanghai Public Health Clinical Center, told Sixth Tone that people should not panic over this new piece of information.

“An incubation period by definition will have outliers,” Shen said. “It’s not a unique problem with the coronavirus, but rather happens with most types of infections. So far, there isn’t enough evidence to change the quarantine policy for all suspected cases.”

Previously, China had reported two isolated cases of COVID-19 with prolonged incubation periods of 27 and 38 days.

“As a doctor, it’s important that we are aware of the issue,” Shen said. “But more importantly, we need to spend most of our time catching the 90% who fall within the normal range instead of fixating on the 10%.”

Editor: David Paulk.

(Header image: An official takes a boy’s temperature in Shulan, Jilin province, May 2020. Liu Zhihong/People Visual)