What’s the Evidence for Covid-19 Reinfection?

A lab worker handling a positive covid-19 sample at the Hermes Pardini Lab in Brazil. (Photo: Pedro Vilela, Getty Images)
A lab worker handling a positive covid-19 sample at the Hermes Pardini Lab in Brazil. (Photo: Pedro Vilela, Getty Images)

A small bright spot in the coronavirus disaster has been that people who become sick appear to have at least temporary immunity to the virus. But over the weekend, a Vox article written by a primary care doctor in Washington D.C. laid out an imminent and frightening pandemic scenario, based on one of his own cases: People coming down with a second, possibly worse bout of covid-19 within months of their first infection.

Evidence for reinfection happening this soon is still very limited, however, and there are many key questions about immunity to covid-19 that need to be studied further before we can really confirm the possibility of reinfection and how it might play out.

The Vox article was written by D. Clay Ackerly, an internal medicine and primary care physician practicing in Washington, DC. In it, he details a 50-year-old patient who reportedly tested positive for the coronavirus that causes covid-19, SARS-CoV-2, twice over a span of three months, the latest in early July. Ackerly dismissed the possibility of the patient having one long infection as unlikely, since the man had tested negative for the virus twice following his first period of symptoms, which were mild; he had also felt perfectly healthy for nearly six weeks. The second time around, the man’s symptoms were much worse, including low oxygen levels and trouble breathing, and he sought care at a hospital multiple times.

“I believe it is far more likely that my patient fully recovered from his first infection, then caught Covid-19 a second time after being exposed to a young adult family member with the virus,” Ackerly wrote.

Ackerly isn’t the first doctor or expert to raise the possibility of reinfection. For months, there have been isolated reports of people from the earliest days of the pandemic testing positive for the virus again following what seemed to be their initial recovery. Notably, South Korean health officials reported on hundreds of these “relapsed” cases within their country as early as April, however, they concluded that the second round of positive samples they had found were likely the result of dead coronavirus still lingering in patients.

Reinfection concerns have been further amplified by studies suggesting that people’s levels of antibodies to the virus generally take a sharp drop within two to three months.

But right now, according to the Columbia University virologist Angela Rasmussen, these concerns aren’t backed up by concrete data.

“As far as I know, there hasn’t been a documented case of reinfection,” she told Gizmodo. “People who have tested positive after recovering have not been associated with new cases, and attempts to culture infectious virus from their samples have all been negative, suggesting that the positive test was detecting residual viral RNA, not bona fide replicating virus.”

This doesn’t mean that covid-19 reinfection isn’t possible. It’s just that the science around how natural immunity to SARS-CoV-2 works is very much unsettled.

Take, for example, those antibody studies mentioned earlier. It could definitely be true that levels of covid-19 specific antibodies drop within months. But it could also be true that people still carry enough of the antibodies most relevant to preventing reinfection — called neutralising antibodies — that their immunity will last longer than a few months. As I’ve written before, immunity to germs isn’t just about antibodies: Our immune system also relies on cells, particularly certain types of T cells, that similarly recognise and go after familiar threats. And research is showing that the T cell response to covid-19 post-infection is robust.

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While we don’t know much about what determines a person’s protective immunity to covid-19, Rasmussen said, a lack of antibodies after a few months does not necessarily mean that a person is vulnerable to reinfection.

Given what we already know about other kinds of coronaviruses that infect people regularly, though, it would be weird if at least some survivors weren’t susceptible to getting covid-19 again at some point. But with these other coronaviruses, studies have suggested that immunity starts to noticeably wane closer to a year or more after the first infection. The possibility of covid-19 being worse the second time is less likely, since reinfection with any germ in general tends to be milder. There is a stacking effect with some diseases, most notoriously dengue fever, and immunocompromised people are likely more at risk of having a second infection that’s just as bad or worse as the first, but both examples are exceptions to the rule.

Another crucial point is that reinfection doesn’t close the door on an effective vaccine. Some vaccines are capable of evoking a better, longer-lasting immune response than the natural infection does, and this is an express goal for the scientists developing covid-19 vaccines. Booster shots are also routinely used to refresh our immunity. Even the least effective vaccines we have, the flu shot in particular, provide a real benefit in reducing the severity and deadliness of their target illness. That said, scientists will be watching closely to figure out how our immune systems respond to any vaccine candidates.

Ackerly’s anecdotal case and others like it are certainly worth keeping an eye on, Rasmussen said. And it’s certainly possible that some segment of covid-19 survivors could lose their immunity that quickly, particularly if their initial immune response wasn’t strong. But she doesn’t rule out the idea that the virus really could have persisted in the patient’s body those past three months, as some viruses can. A third possibility could be a false positive the first time around (Ackerly doesn’t state whether the first infection was confirmed with more than one positive result; he also notes that his patient wasn’t able to get tested for antibodies). Without more evidence, though, Rasmussen is not convinced that these sorts of cases are broadly applicable to the general population or that we need to be worried about widespread reinfection.

Unfortunately, the U.S. still is very much in the grips of the first wave of covid-19, so for most people, the initial infection remains the greatest risk.