Covid-19 : Immunity in question

Reports of reinfection with the coronavirus evoke a nightmarish future: Repeat bouts of illness, impotent vaccines, unrelenting lockdowns — a pandemic without an end.

Reinfection with the coronavirus is an unusual event

Reinfections can occur for any number of reasons: because the initial infection was too mild to produce an immune response, for example, or because the immune system was compromised by other health conditions. On occasion, a patient may be exposed to a large amount of virus that seeded an infection before the immune response could respond. This variability is entirely expected, experts said, and has been observed in patients with diseases like measles and malaria. “You’ll never have the distribution of anything with millions of people where you don’t have some very severe rare cases happening at the fringe,” said Dr. Michael Mina, a pediatric immunologist at the Harvard T.H. Chan School of Public Health.

A resurgence of symptoms doesn’t prove reinfection

For every confirmed case of reinfection, there are dozens of anecdotal reports of infected people who were sick and seemingly recovered but then became ill again weeks to months later. Usually there are crucial data missing in those cases, like a confirmed lab diagnosis, or a virus sample that can be sequenced.

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RA report from Hong Kong provides the first credible evidence that it’s possible to catch the virus twice. Some immunologists are breathing a sigh of relief.

A case of reinfection is not necessarily the same thing as a lack of protection

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.

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Debates over herd immunity and natural infection arise with every outbreak. Effective vaccination always wins.

What we mean when we talk about herd immunity

Yet when most people discuss herd immunity today, they’re really talking about what’s known as the “herd threshold theorem.” It’s what scientists are referencing when they say 75 percent of the population needs to be immune against COVID-19 to stop disease transmission, and it’s surprisingly simple to calculate. Say a germ lands in foreign world, where an entire population is susceptible. And say it becomes clear that one infected person will transmit it to four others on average—a value known as the germ’s basic reproduction number, represented by an R with a subscript zero and thus called R-naught. To flatten the outbreak’s growth, you want a situation where the afflicted can infect just one person out of four.

Our future with COVID-19 depends on us

On August 14, Tom Britton, a mathematician at Stockholm University in Sweden, and two other scientists released a model in Science that estimates how social activity might influence the herd immunity threshold. They started with the valid assumption that millennials and Gen Z mix more than older people, and so will more readily spread the virus. Britton’s team landed on a herd threshold of 43 percent—much lower than the 60 to 75 percent you get using the classic equation.

“We don't claim that the number from our model applies in reality,” Britton cautions, adding that the model merely shows the degree to which disease-induced immunity can play a role. “We don't want our paper to have the consequence that people feel relaxed and say, Let's skip restrictions and wait for herd immunity.”

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