A man in Hong Kong is the first confirmed case of coronavirus reinfection

A 33-year-old man in Hong Kong was infected with the coronavirus a second time, more than four months after his initial infection, researchers report. His case is 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.

“The key unknowns at the moment are how often this occurs and to what extent,” Bieniasz says.

If reinfections are relatively common, it could make reaching herd immunity — the proportion of the population that has to be immune to protect other people — through natural infections more difficult (SN: 3/24/20). Vaccines, however, might trigger a more robust immune response and help protect populations by providing herd immunity.

Overall, it’s unknown how long immunity to the coronavirus lasts. Some people can test positive for the virus’s genetic material for months after their recovery, but do not shed infectious virus (SN: 5/19/20). A few studies measuring antibodies — key immune proteins that recognize and bind to pathogens — suggest that antibody levels do wane over time. Other preliminary work hints that antibodies that can stop the virus from entering cells remain in the blood for at least three months.

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, but is not yet available online.

The first time the man was infected, he had a fever, cough, sore throat and headache for three days. He tested positive for the coronavirus on March 26 and was admitted to a hospital in Hong Kong three days later. At that point his symptoms had gone away. He was discharged on April 14 after he tested negative for the virus twice. But the man tested positive again more than four months later on August 15, when officials screened him upon returning to the Hong Kong airport from Spain. The second time, he never showed any indication of being sick — a sign that his immune system kicked in enough to protect him from the disease because he remained asymptomatic but not enough to prevent reinfection.

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.

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.

Although the study reveals that reinfection is possible, it also suggests that the immune system is doing its job to fight off infections, researchers say. And “people shouldn’t think that everybody’s going to be susceptible to infection [again] within four months,” Bieniasz says. Because the man originally tested negative for antibodies during his first infection and researchers did not monitor levels over time, “we don’t really know what the magnitude of the immune response was in this person.”

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.”

It’s also unknown whether the man could transmit the virus to others. “That’s an important question,” says Kartik Chandran, a virologist at Albert Einstein College of Medicine in New York City. Some people might still develop some sort of infection a second time, he says, and if they mount a strong immune response, they may not get seriously ill, but they may still shed the virus and infect other people. But it’s also possible that the immune response to a new infection could lower the amount infectious virus a person is capable of transmitting to someone else.

For now, researchers need to evaluate more cases, experts say, to see if this man’s case was more the norm or an exception. “If you infect 5 million people with anything, there’s going to be one or two people that are way different than the rest,” Slifka says.