Researchers at the University of Hong Kong have recorded what they said was the first genetically proven case of Covid-19 reinfection, four and a half months after the patient originally caught the virus.

The 33-year-old man was hospitalised with moderate Covid-19 symptoms in Hong Kong in March, then tested positive again in August when he was screened at Hong Kong airport on returning from a trip to Spain. On the second occasion he displayed no symptoms of infection.

The researchers proved that he had been reinfected — and that the coronavirus had not simply remained in his body — by taking genetic fingerprints on each occasion. These showed that the second virus had 24 differences from the first — more than could have occurred through mutations within one individual.

Although the findings suggest that immune protection from the Sars-Cov-2 virus that causes the disease may not last very long, other scientists responded cautiously to the report, which is scheduled for publication in Clinical Infectious Diseases but has not been described fully in a scientific journal.

Maria Van Kerkhove, technical lead for Covid-19 at the World Health Organisation, said she was still studying the Hong Kong reinfection case but urged people to put it into the context of 24m cases reported worldwide.

“I don’t want people to be afraid,” she said. “We need people to understand that if they are infected, even if it is only a mild case, they do develop an immune response.”

Sars-Cov-2 reinfection has been reported previously but those cases were not accompanied by genetic evidence to exclude the re-emergence of an existing infection.

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Brendan Wren, professor of microbial pathogenesis at the London School of Hygiene & Tropical Medicine, said: “It is to be expected that the virus will naturally mutate over time. This is a very rare example of reinfection and it should not negate the global drive to develop Covid-19 vaccines.”

Others drew reassuring conclusions from the case, because the second infection was asymptomatic — suggesting that the patient’s immune system might have prevented the disease developing even though it did not stop the virus entering his body.

“It is very hard to make any strong inference from a single observation,” said Jeffrey Barrett of the Wellcome Sanger Institute’s Covid-19 Genome Project. “It may be that second infections, when they do occur, are not serious, though we don’t know whether this person was infectious during their second episode.”

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Via Financial Times