Throughout the Covid-19 pandemic , scientists have been saying that if the novel coronavirus, SARS-CoV-2, sticks around long enough, people are likely to catch it more than once. That’s based on what immunologists know about other members of the coronavirus family—the ones that have long survived their initial spillover events and now circulate seasonally , causing the common cold. People who get sick mount an immune response that protects them for months or years, depending on the person. But, at some point, that protection wanes and they become susceptible to infection again.
On Monday, researchers at Hong Kong University presented the first confirmation that this can, in fact, happen with SARS-CoV-2. Not a shock, say experts. But still a useful data point for understanding how immunity to the coronavirus works, both in individuals and in populations. At the molecular level of antibodies and T cells, the case provides reason to be hopeful. As for the odds of achieving herd immunity without a vaccine? It’s a cautionary example.The report details how in March, a 33-year-old man living in Hong Kong came down with a sore throat, cough, fever, and headache. Tests confirmed he was positive for the virus. After two weeks in the hospital, his symptoms subsided and he was discharged. He resumed his life. And over the summer, he traveled to Spain. In August, on his way back, he was swabbed at the Hong Kong airport, as part of the nation’s strict traveler-screening process designed to catch any reimportations of the virus. He had no fever or cough, no symptoms at all. But the test came back positive again.
This means that the herd immunity threshold will also be higher than 60 percent in some places and lower in others.“I think the range of R0 consistent with data for Covid-19 is larger than most people give credit to,” said Marc Lipsitch of Harvard University, who has been advising health officials in Massachusetts and abroad.
When Hong Kong University scientists compared the viral genomes extracted from each swab, they discovered significant differences between them. Both were SARS-CoV-2. But their genetic fingerprints didn’t match. The virus that infected the man in March was most closely related to strains collected in the USA and England during the spring. The one found inside his body in August bore the most resemblance to strains circulating around Europe in late summer. The only plausible explanation, the researchers concluded, was that four and half months after his first bout with Covid-19, he’d been infected a second time.
Sounds bad? Sure. But to those who study the immune system, a case of reinfection is not necessarily the same thing as a lack of protection. “This is actually good news, that this person was protected from illness,” says Donna Farber, an immunologist at Columbia University. It’s common for respiratory viruses to infect people more than once—and Farber was unsurprised to see it occur with SARS-CoV-2. That’s all part of the natural course of building immunity over time, akin to receiving a booster shot after a vaccination. But what’s important, she says, is that the second time around the immune system appears to have done its job and cleared the virus with little drama. That’s immunity at work. “Normally this wouldn’t even register, because this guy didn’t get sick. You wouldn’t even see it,” she says.
It’s difficult, however, to extrapolate from a single case. Scientists have been unsure what to make of the world-first, in part because of the way the news was disseminated. The study’s results were first publicized in a Hong Kong University press release Monday, and a subsequent story in the South China Morning Post. WIRED obtained a copy of the manuscript, which was later published online in the journal Clinical Infectious Diseases. Yuen Kwok-yung, one of the researchers who led the work, told WIRED in an email that the journal’s chief editor allowed the scientists to communicate with the media once the paper was accepted but ahead of publication, due to the public health significance of the finding that recovered people can be reinfected.