A growing number of discharged coronavirus patients in China and elsewhere are testing positive after recovering, sometimes weeks after being allowed to leave the hospital, which could make the epidemic harder to eradicate.

On Wednesday, the Osaka prefectural government in Japan said a woman working as a tour-bus guide had tested positive for the coronavirus for a second time. This followed reports in China that discharged patients throughout the country were testing positive after their release from the hospital.

An official at China’s National Health Commission said on Friday that such patients have not been found to be infectious.

Experts say there are several ways discharged patients could fall ill with the virus again. Convalescing patients might not build up enough antibodies to develop immunity to SARS-CoV-2, and are being infected again.

The virus also could be “biphasic”, meaning it lies dormant before creating new symptoms.

But some of the first cases of “reinfection” in China have been attributed to testing discrepancies.

On February 21, a discharged patient in the southwestern Chinese city of Chengdu was readmitted 10 days after being discharged when a follow-up test came back positive.

Lei Xuezhong, the deputy director of the infectious diseases centre at the West China Hospital, told People’s Daily that hospitals were testing nose and throat samples when deciding whether patients should be discharged, but new tests were finding the virus in the lower respiratory tract.