An increasing number of recovered coronavirus patients in China are continuing to test positive for the virus without showing any symptoms, according to a report from Reuters.

Doctors in Wuhan, China, where the COVID-19 outbreak began late last year, said the patients tested negative at some point during their recovery, but then began testing positive again without symptoms. Some were testing positive again as much as 70 days after being cleared.

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Data from Reuters and other media outlets indicate there are at least dozens of such cases, but China has not published exact figures on how many recovered patients have tested positive again. Chinese officials say there have been no confirmations of newly positive patients infecting others.

Similar reports have been coming out of several other countries, including South Korea, Japan and Italy.

About 1,000 people in South Korea have reportedly tested positive for four weeks or more, and in Italy, health officials said patients could test positive for more than a month.

“We did not see anything like this during SARS,” Yuan Yufeng, a vice president at Zhongnan Hospital in Wuhan told Reuters, speaking on a case in which a patient had positive re-tests after first being diagnosed with the virus about 70 days earlier.

Health experts have previously noted the possibility of testing errors and released patients from hospitals too early as a more likely cause of recovered patients testing positive again, but the ongoing reports of recovered patients testing positive underlines how much is unknown about COVID-19.

The World Health Organization (WHO) confirmed earlier this month it is investigating such incidents in response to the reports out of South Korea.

“We are closely liaising with our clinical experts and working hard to get more information on those individual cases. It is important to make sure that when samples are collected for testing on suspected patients, procedures are followed properly,” the WHO said.

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