On Aug. 24, 2020, Hong Kong University researchers reported that a 33-year-old man in Hong Kong had been infected with the coronavirus for a second time, more than four months after his initial infection. His case was the first confirmed account of SARS-CoV-2 reinfection.

The fact that some people can be reinfected with the virus is “not a huge shock,” says Paul Bieniasz, a virologist at the Rockefeller University in New York City and with the Howard Hughes Medical Institute. People often get reinfected with the coronaviruses that cause the common cold. And some people may not mount a strong enough immune response to fight off SARS-CoV-2 a second time.

Previous anecdotal reports of patients who had recovered from an infection only to be sickened again with COVID-19 months later have surfaced during the pandemic. But without genetic evidence that each round of illness was caused by two distinct viruses, it was unclear whether such cases were true reinfections.

Genetic data confirmed on August 24 that the man was in fact reinfected, 142 days after his first illness, the researchers reported August 24. A study describing the findings has been accepted for publication in Clinical Infectious Diseases.

In the new study, Kwok-Yung Yuen, a microbiologist at the University of Hong Kong, and colleagues analyzed most of the genetic blueprint, composed of RNA, of each of the patient’s coronavirus infections and looked for differences. Viruses from each bout of COVID-19 differed by 24 nucleotides, the building blocks that make up the virus’s RNA. Those differences indicate that the man was infected two different times by two versions of SARS-CoV-2: one that is closely related to variants circulating in the United States and England in March and April, and another related to viruses from Switzerland and England in July and August.

It’s unclear how often reinfections with the new coronavirus happen globally, especially if the second round of infection results in milder symptoms. “When we say that people don’t typically get reinfected, that’s based on the epidemiology of when people get sick again,” says Mark Slifka, a viral immunologist at Oregon Health and Science University in Portland. “This might happen more often than we think, but no one’s swabbing people on a routine basis when they’re healthy.”

Ten days after the patient developed symptoms the first time, he tested negative for antibodies against the virus. But it may have been too early for researchers to detect the immune proteins. The second time around, he also didn’t have antibodies in his first three days in the hospital. He tested positive for the immune proteins on the fifth day, a sign his immune system had rapidly mobilized to fight the virus.