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“It is unlikely that herd immunity can eliminate SARS-CoV-2,” he and his coauthors wrote in the report. “Although it is possible that subsequent infections may be milder than the first infection, as for this patient.”

Throughout the pandemic, doctors and researchers around the world have documented a handful of presumed Covid-19 reinfections. In the first case, a woman in Japan fully recovered before returning to the hospital three weeks later with a new round of symptoms. In other instances—in China, France and the US—patients went up to six weeks between negative tests and a new positive one. But those reports, which lacked systematic genetic analysis, were largely believed to be the result of flawed testing or unusually long-lived infections flaring up again. Some recovered patients can harbor the virus’s genetic material inside their nose and throats for months, long after their symptoms have subsided. That can lead to positive test results even in the absence of an active infection.

“People have assumed these reports were not true reinfections, but cases of prolonged viral shedding,” says Susan Kline, an infectious disease physician and epidemiologist at the University of Minnesota. This Hong Kong case, on the other hand, looks like the real deal to her. “The evidence here, with the sequences of the virus, is quite strong that this patient really was infected with a different strain the second time,” she says.

Reinfection is possible, yes. But just how common is it? That’s still unclear. Few swabs taken from patients are later used to extract a full viral sequence, enabling this kind of genetic detective work. And regular screening of people with no symptoms only happens in nations that have largely kicked Covid-19. In the US, where access to rapid testing is still hit-or-miss, such screening is often only conducted for health care workers and inside nursing homes and some prisons. “I suspect there are likely more patients like this out there,” says Kline.

For now, there’s just the one. That makes it impossible to say what reinfection will look like in other people. But at the time of his second infection, it appears this particular person lacked what immunologists call “sterilizing immunity,” says John Wherry, an immunologist at the University of Pennsylvania. That’s when a person’s immune system is able to completely stifle the virus’s ability to replicate, meaning no infection occurs at all.

In reality, Wherry says, “that is a very high bar to set.” The second time the body encounters a virus, it has a head-start against intruders: neutralizing antibodies, which glom onto specific proteins on the invading pathogen and prevent it from entering cells. Usually, those antibodies stick around at some level after an initial infection or shot of a vaccine. But it’s an imperfect defense system. Even the best vaccines protect only 90 to 95 percent of the population from reinfection, Wherry notes. Similarly, natural infections by respiratory viruses almost always leave some chance of reinfection. Maybe the second time around there aren’t enough antibodies on hand—they’ve waned, perhaps, or the body didn’t mount much of a response in the first place. Or maybe those antibodies are not perfectly adept at plugging up the proteins the virus uses to latch onto a cell. In that case, some virus winds up getting inside and begins to replicate. A new infection has begun.

What turns an infection into an illness largely depends on what comes next. During the primary infection, the body has never seen the virus before, so it’s stuck creating a customized immune response from scratch: an army of B cells that help create antibodies specific to the virus’s proteins, and T cells that help identify virus particles and eliminate infected cells. But while those forces are being mustered, the virus has an ample window of opportunity—perhaps a week to 10 days, Wherry estimates—to replicate and spread. “By then, the virus has spread around the lungs and outside the lungs, and when the troops show up you’ve got a big problem on their hands,” he says.

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