Several COVID-19 patients in China and other countries were testing positive again after recovering, raising questions about the containment efforts. The reappearances were sometimes noted merely weeks after the patients were discharged from hospitals.

An unidentified Japanese woman from Osaka was tested positive again for the virus after recovering. She contracted the disease in January and was released from hospital earlier this month. The virus appeared in her again this week, the Osaka Prefectural Government said on Feb. 26. The woman worked as a guide on a bus carrying tourists from the Chinese city of Wuhan, the epicenter of the outbreak.

In China, many people were reportedly tested positive again their release from hospitals. China's National Health Commission on Friday has officially declared that such patients were not found to be infectious.

Discharged patients, however, can be vulnerable to a second infection shortly after recovery because they may not build enough antibodies to aid their immunity to the virus. The virus could also be "biphasic," which means lying dormant before triggering new symptoms, Reuters reported, citing experts.

However, the “reappearance” of the virus in many discharged patients in China was nothing but testing discrepancies. A patient in the city of Chengdu was readmitted to the hospital 10 days after the release when a follow-up test indicated him positive for the virus.

Lei Xuezhong, deputy director of the infectious diseases center at the West China Hospital, told local publication People’s Daily that hospitals were testing nose and throat samples of patients to decide whether they were eligible to be discharged. However, fresh tests indicated the virus was still present in the lower respiratory track.

Paul Hunter, a professor of medicine at Britain's University of East Anglia, who has been keeping a keen eye on the outbreak, told Reuters that virus from her previous infection was possibly releasing in the body of the Osaka victim, and she was not tested properly before her release.

However, some experts speculated that the virus in her was biphasic, looking at the short time gap between her discharge and readmission.

Some patients were reportedly said to still remain carriers even though doctors deemed them eligible for discharge. Experts were grappling to understand whether the patients got infected again or remained positive all the while.