Post-infection immunity might be a strange topic for political strife, but it touches on COVID vaccine mandates and whether those who’ve had the virus should be exempt. And so when publications such as The Hill run headlines such as “CDC Finally Recognizing Natural Immunity – Legislators should Follow,” it carries the implication the CDC has been ignoring some long-held scientific evidence. But the science is more complicated and unsettled than that.
The relevant science question isn’t whether natural immunity exists but whether it’s as protective and lasts as long as vaccine-induced immunity. Studies have given conflicting answers. The situation is now changing again, as the BA.2 variant is starting to take over. It’s still considered omicron, but it looks to be wildly different from the version of omicron that’s been dominant, called BA.1.
Scientists confirmed that an infection with SARS-CoV-2 creates some degree of immunity in May 2020, based in part on a study published in Science, which was led by Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center. “We did the study in May 2020, not so much to test natural immunity, but really as part of our vaccine program,” he told me this week.
Vaccines work by fooling the body into acting as if it’s been infected, and so vaccines work best against diseases where post-infection immunity is strong and lasting, he said, such as measles, mumps and rubella. It hasn’t worked for HIV, a virus that attacks the immune system and can’t be cleared except in some rare cases with medical intervention.
With COVID, antibodies wane over time. That can decrease immunity, as can new variants. With the original variant, it looked like vaccines gave better protection than getting infected. But during the delta surge, some studies showed that had reversed. New data released by CDC show that a previous infection gave people better protection against delta than either one or two shots of a vaccine. But getting vaccinated in addition to getting infected gave people the strongest protection — so it wasn’t overkill to recommend vaccination to people who’d been infected. Boosters also restored most of the effectiveness of the vaccines.
“There was an early perception that vaccination is hands down better than natural infection — and a lot of people still feel that will be the case,” Barouch said. “But I think it’s more nuanced now, and a lot of people think natural immunity gives you a substantial level of protection.” But how much protection, compared to vaccination? “That’s not clear.” Barouch agrees with many other researchers I’ve talked to — that a confirmed COVID infection could probably substitute for a single shot.
Before the COVID-19 vaccines were developed, scientists told me that with other viral infections, vaccines are sometimes more protective than past infection and sometimes less so. One reason vaccines sometimes work better is that viruses often disable a person’s immune response as part of their own survival strategy. That’s happening with SARS-CoV-2 according to some studies, said Shiv Pillai, a professor of immunology at Harvard Medical School.
The disadvantage for COVID-19 vaccines is that they are designed to only produce antibodies that attack just one part of the virus — the spike protein. As the spike protein keeps mutating, the vaccines lose some efficacy. Getting infected might provide people with broader, multi-variant protection.
It seems absurd for anyone to try to get infected as a way to gain protection against getting infected, but it’s possible some people might try to get infected if they are avidly anti-vaccine, and infection could satisfy a vaccine mandate or passport system. Vaccines are vastly safer than getting the virus, but not everyone is well informed or thinking clearly.
Omicron’s heavy mutations changed the game. The vaccines still offer good protection against severe disease, but neither vaccination nor infection with an earlier variant offers much protection against a mild omicron infection. This post by Eric Topol, a professor of medicine at Scripps Research, ties together a lot of studies, but he also points out that most of the studies comparing vaccine-induced immunity to past infection involve the original variant or delta. Now the pandemic is nearly all omicron — a strikingly different variant that's much better at escaping both kinds of immunity — and there's much less known.
The new BA.2 version of omicron is forcing scientists to recalculate — yet again. Barouch has been leading some of the first experiments testing how well BA.2 slips by defenses from past infection or vaccines. “It’s slightly worse, but not a ton worse,” he told me. It looks like omicron infection protects to an extent against BA.2, so the experts think it’s unlikely to lead to the kind of massive infection wave caused by the original omicron. But there are scores of other unexpected things that could happen — good or bad — with this ever-changing pandemic.
Policy decisions about vaccine mandates can be informed by science but can’t be determined by science. Those decisions depend on the ethics and legality of mandating vaccines, and whether staying unvaccinated harms society by increasing transmission or depleting precious hospital resources.
Policy aside, severely immune-compromised people are already being advised to get a second booster. Right now, there’s no recommendation for an additional booster for the general public — doctors have told me it’s rare to see fully boosted people be hospitalized for COVID. But that might change in the coming months, if there’s a new wave, or evidence that immunity is fading. A past infection might play into that decision. (For those wondering whether they’ve been infected, it’s possible to get a test that would pick up antibodies not generated by the vaccine, but those aren’t widely available.)
Such decisions hinge on which new variants emerge in the future, ongoing research into the duration of immunity, and whether scientists eventually develop a more variant-proof coronavirus vaccine. The last two years may have gone by slowly, but we’re still dealing with a new disease, and there’s a lot yet to learn.
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ABOUT THE WRITER
Faye Flam is a Bloomberg Opinion columnist and host of the podcast "Follow the Science." She has written for the Economist, the New York Times, the Washington Post, Psychology Today, Science and other publications.
This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.
