Researchers in Hong Kong are reporting the first confirmed case of reinfection with the coronavirus.

“An apparently young and healthy patient had a second case of COVID-19 infection which was diagnosed 4.5 months after the first episode,” University of Hong Kong researchers said Monday in a statement.

The report is of concern because it suggests that immunity to the coronavirus may last only a few months in some people. And it has implications for vaccines being developed for the virus.

The 33-year-old man had only mild symptoms the first time and no symptoms this time around. The reinfection was discovered when he returned from a trip to Spain, the researchers said, and the virus they sequenced closely matched the strain circulating in Europe in July and August.

“Our results prove that his second infection is caused by a new virus that he acquired recently rather than prolonged viral shedding,” said Dr. Kelvin Kai-Wang To, a clinical microbiologist at the University of Hong Kong.

Given that there are millions of cases worldwide, it is not unexpected that a few, or even a few dozen, people might be reinfected with the virus after only a few months, experts have said.

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Doctors have reported several cases of presumed reinfection in the United States and elsewhere, but none of those cases have been confirmed with rigorous testing. Recovered people are known to shed viral fragments for weeks, which can cause tests to show a positive result in the absence of live virus.

But the Hong Kong researchers sequenced the virus from both rounds of infection and found significant differences in the two sets of virus, suggesting that the patient was infected a second time.

Common cold coronaviruses are known to cause reinfections in less than a year, but experts had hoped that the new coronavirus might behave more like its cousins severe acute respiratory syndrome and Middle East respiratory syndrome, which seemed to produce longer-lasting immunity of a few years.

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