About 14 per cent of patients who recovered from the coronavirus and were discharged from hospitals in southern China's Guangdong province were tested positive again in later check-ups, according to the local health authority.

A positive test suggests that the recovered patients may still carry the virus, adding complexity to efforts to control the outbreak.

There is no clear conclusion on why it happens and whether such patients could still be infectious, Dr Song Tie, deputy director of the Guangdong Centre for Disease Control and Prevention, said at a Tuesday briefing. According to preliminary assessment, experts believed the patients were still recovering from lung infections and had yet to be fully healthy, he said.

Under the latest treatment guidelines for Covid-19 issued by the National Health Commission, patients can be considered recovered and released from hospital when their throat or nose swabs show up negative in two consecutive tests, with a CT scan indicating no lung lesions, and when they have no obvious symptoms such as fever.

The guidelines suggest that recovered patients should monitor their health and limit outdoor activities for two weeks after leaving the hospital, and check in for re-testing in the following weeks.

Some patients' test results returned to positive in the follow-up checks, Dr Li Yueping, director of the intensive care unit at Guang-zhou No. 8 People's Hospital, said at the briefing. The hospital found 13 discharged patients who tested positive again, although none showed renewed symptoms.

Nucleic acid tests for 104 close contacts of the patients all found negative results, added Dr Li.

Dr Cai Weiping, director of the Infectious Diseases Division of the No. 8 People's Hospital, said the positive results in the recovered patients were all found from anal swabs, a method rarely used in other parts of the country.

Dr Song said the province is preparing to put the recovered patients who re-tested positive under close observation. The health authorities will also step up the monitoring of discharged patients and their condition as they recover, he added.

Similar cases of recovered patients testing positive have been reported elsewhere in China. Last week, a patient discharged after recovering in Sichuan province's Chengdu city was readmitted after testing positive again. The health authorities in Hainan also confirmed that some recovered patients had tested positive again.

• This story was originally published by Caixin Global.