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A Hong Kong man who was initially infected with the coronavirus in March and made a full recovery was reinfected more than four months later after a trip abroad, researchers reported Monday.

The pre-print study, by a team at the University of Hong Kong, purports to be “the world’s first documentation” of a patient who recovered from covid-19 becoming reinfected. Researchers sequenced the genome of his first and second infections to show the virus strains were different, suggesting he had been reinfected,

The findings, which have not been peer-reviewed, have potential implications for vaccine use, as well as policies based around the concept of herd immunity that presume those who recover from the virus are unlikely to be reinfected. While there have been many anecdotal reports of reinfection, until now, they have not been confirmed and it remains unknown how often they might occur.

The fact that the man had no symptoms the second time suggests his immune system protected him from disease, although it did not stop the reinfection.

Study author Kwok-Yung Yuen and his colleagues suggest in their paper that herd immunity is unlikely to eliminate covid-19 on its own and that a potential covid-19 vaccine may not provide lifelong immunity to the disease.

But some immunologists emphasized the case was not a surprise and offered a more positive interpretation of the findings. “This is a textbook example of how immunity should work,” Akiko Iwasaki, an immunology expert at Yale University, tweeted on Monday, referring to the man’s lack of symptoms.

Iwasaki also said the research had “no bearing” on the success of a potential coronavirus vaccine. “Vaccines can provoke a much higher level of immunity in a person that can potentially block reinfection, or at least shut it down to a noncontagious level,” she wrote in response to a reporter’s questions.

The study was accepted the Clinical Infectious Diseases journal on Monday and it is likely to be published online within the next day, said Lili Kadets, a journal spokeswoman. It was “reviewed and accepted” by Robert T. Schooley, an infectious-diseases expert at the University of California at San Diego, and the journal’s editor in chief in what Kadets called a "rapid review process” used for some covid-19 studies due to their time-sensitive nature.

The Hong Kong man whose case was studied, an unnamed 33-year-old IT worker with a reported history of good health, had first tested positive for the coronavirus in late March. His symptoms included a fever and a cough and he was sent to the hospital in keeping with city guidance.

The man was released in mid-April after testing negative for the virus and having no further symptoms. But after visiting Spain via Britain in August, he tested positive again upon returning to Hong Kong, despite appearing asymptomatic.

Physicians at first thought he might be a persistent carrier of the virus, the study’s authors write, but they sequenced the genome of his first and second infections to show the virus strains were different, indicating he had been reinfected.

The man’s reinfection may suggest the level of immunity after an infection may be lower than many had hoped, or may decline over time, or may occur on a spectrum, for instance, offering partial immune protection.

The paper described that while the patient had no detectable antibodies at the time of the second infection, he developed them afterward — a sign Iwasaki called “encouraging.”

Studies that look at antibodies after a coronavirus infection have reached differing conclusions about the level of immunity that can be expected months later. Experts have noted with the common cold, another illness caused by a coronavirus, people are often reinfected each season.

“What we are learning from this new case report is that SARS-CoV-2 may persist in the global population, similar to other common cold-associated human coronaviruses, even if patients have some degree of acquired immunity,” Robert Glatter, an emergency physician at Lenox Hill Hospital in New York City said in an interview.

“The only safe and practical approach to achieve herd immunity is through vaccination,” he said, although even immunizations might not provide lifelong protection.

Glatter said that more studies are needed to “evaluate the spectrum of illness — and degree of immunity achieved — as a result of reinfection” by the virus. It will be key to figure out how widespread reinfection might be and whether subsequent infections carry as many risks as the first infection.

“The take home is that a vaccine, no matter how novel or sophisticated, may not be able to provide lifelong immunity against covid-19. This is in keeping with other seasonal coronaviruses,” he said, adding that it may be possible that vaccines have to be administered twice a year to supplement natural immunity.

The Hong Kong study’s authors, for their part, recommend that those who have recovered from the coronavirus continue to comply with social distancing and other protocols, such as wearing a mask.