Fears are growing that patients who recover from the coronavirus may not have immunity, after South Korea revealed that increasing numbers of people are testing positive for a second time.

Some 163 patients there have been found to have Covid-19 again, having previously being discharged following a first bout.

The findings raise the disturbing possibility that contracting the infection once does not guarantee a natural defence in the future.

Speaking at a press conference on Friday, Kwon Joon-wook, deputy director of the Korea Centres for Disease Control and Prevention, said authorities would now test 400 people who had been infected, with the aim of determining specifically how much – if any – immunity they appear to have.

However, he emphasised there was no concrete evidence that patients who had tested positive twice had actually contracted the virus again.

Rather, officials said that, while possible explanations are still being investigated, they suspect it may simply be a sign that the original virus had not disappeared from the body entirely: the hyper-sensitive tests may have detected fragments of the destroyed virus’s RNA.

“That’s one possible and very strong explanation,” Kwon said.

The same theory was posited by Zhong Nanshan, one of China’s top respiratory experts, earlier in the week.

In a press conference, he said that a recovered person can test positive because elements of deactivated disease remained in their body. “I’m not too worried about this issue,” he added.

Both scientists suggested early indications were that such fragments, while potentially causing minor symptoms in the patient, would probably not be contagious to others.

Errors in testing were also mentioned as a possible cause – although, given the number of people found to be positive for a second time, this seems increasingly unlikely.