A senior epidemiologist said Friday that the complete gene sequence of the novel coronavirus has been detected in a small number of recovered patients, suggesting that they could still be infectious.

But experts are yet to separate a complete virus from such patients, said Wang Guiqiang, head of infectious medicine at Peking University First Hospital in Beijing.

He told a daily news conference in Beijing that such cases are rare, but added that isolation and medical observations are essential for them.

The remarks came as nucleic acid tests involving several recovered patients in Guangdong, Sichuan and Hubei provinces produced positive results, raising concerns that the threshold for discharging patients is low.

To ward off such risks, Wang said the March 3 update in the treatment guideline has required doctors to collect samples from the lower respiratory tract - where the virus is likely to persist - for nucleic acid tests, after some negative results were found to be caused by incorrect samples.

It also required recovered patients to be tested twice, and two negative results can authorize their release, he said.

The released patients are also obliged to go through a 14-day self-quarantine, during which they are advised against sharing food with family members and are required to live in a room with an independent ventilation system.

Currently, the percentage of recovered patients who test positive again stands at 0.1 percent, official figures show. No recovered patients were reported to have infected others so far.

Wang said the numbers are based on current available samples and are not final. The final statistics can only be calculated at the end of the epidemic.