Two patients in China that recovered from COVID-19 months ago tested positive for the coronavirus again, highlighting concerns over the possibility of reinfection.

A 68-year-old woman in the central Chinese province of Hubei, where the novel coronavirus first surfaced in December, tested positive on Sunday — six months after she was diagnosed with COVID-19 and recovered. Another man, diagnosed with the disease in April, tested positive in Shanghai on Monday.

None of the patients’ close contacts has tested positive for the virus, but they have been placed under quarantine.

The phenomenon raises questions over if any immunity to the disease might be too ephemeral to protect against re-infection.

Some studies have shown the level of protective antibodies an infected person may build up to fight the virus quickly drop after only a few months, possibly making them susceptible to the same pathogen a second time. 

However, there is little evidence so far that re-infection has been occurring in this pandemic.

Meanwhile, researchers in South Korea have suggested that the virus detected in patients months after recovery could be the vestiges of dead virus particles that are no longer infectious.

According to the Johns Hopkins School of Public Health website, the possibility of reinfection is low. Researchers are currently trying to understand how to measure a person’s immunity against the virus using antibody tests.

The novel coronavirus has so far sickened more than 20 million worldwide and killed 748,000.