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HONG KONG (KXAN) – A 33-year-old man in Hong Kong has become the first documented case of re-infection of the novel coronavirus, researchers in the area reported on Monday.

The man was previously infected just over four months ago and researchers say his second infection is asymptomatic.

“The second infection was completely asymptomatic — his immune response prevented the disease from getting worse,” said Akiko Iwasaki, an immunologist at Yale University who independently reviewed the report for The New York Times. “It’s kind of a textbook example of how immunity should work.”

Iwasaki said that in order for herd immunity to occur, an effective vaccine would need to first induce immunity that prevents reinfection and disease — which she says happened in this case.

The possibility of reinfection has been reported but not confirmed, however, the Hong Kong researchers’ data shows differences in the viral sequences taken from the patient — signaling a new infection.

“I believe this is the first reported case that is confirmed by genome sequencing,” said Dr. Kelvin Kai-Wang To, a clinical microbiologist at the University of Hong Kong.

Dr. To says the research shows that the virus versions prove the man was reinfected recently, rather than the possibility that the infection is a holdover from the first infection, or “viral shedding.”

Researchers in Hong Kong say the man tested positive for COVID-19 after a trip to Spain from the United Kingdom, where he picked up a strain that was making its way around the country.

Epidemiologist at Columbia University, Jeffrey Shaman, says the news should not cause “undue alarm” just yet.

“However, it remains very, very concerning — and this does nothing to dispel that — that we may be subject to repeat infection with this virus,” he told NYT.

Iwasaki is hopeful, however, saying that the discovery is proof that immunity is happening.

“When you have second exposure to the same pathogen, you should elevate the antibody, and that’s what’s happening.”