Published September 13, 2021 in the British Medical Journal by Jennifer Block

The US CDC estimates that SARS-CoV-2 has infected more than 100 million Americans, and evidence is mounting that natural immunity is at least as protective as vaccination. Yet public health leadership says everyone needs the vaccine. Jennifer Block investigates

When the vaccine rollout began in mid-December 2020, more than one quarter of Americans—91 million—had been infected with SARS-CoV-2, according to a US Centers for Disease Control and Prevention (CDC) estimate.1 As of this May, that proportion had risen to more than a third of the population, including 44% of adults aged 18-59 (table 1).

Estimated total infections in the United States between February 2020 and May 2021*

The substantial number of infections, coupled with the increasing scientific evidence that natural immunity was durable, led some medical observers to ask why natural immunity didn’t seem to be factored into decisions about prioritising vaccination.234

“The CDC could say [to people who had recovered], very well grounded in excellent data, that you should wait 8 months,” Monica Gandhi, an infectious disease specialist at University of California San Francisco, told Medpage Today in January. She suggested authorities ask people to “please wait your turn.”4

Others, such as Icahn School of Medicine virologist and researcher Florian Krammer, argued for one dose in those who had recovered. “This would also spare individuals from unnecessary pain when getting the second dose and it would free up additional vaccine doses,” he told the New York Times.5

“Many of us were saying let’s use [the vaccine] to save lives, not to vaccinate people already immune,” says Marty Makary, a professor of health policy and management at Johns Hopkins University.

Still, the CDC instructed everyone, regardless of previous infection, to get fully vaccinated as soon as they were eligible: natural immunity “varies from person to person” and “experts do not yet know how long someone is protected,” the agency stated on its website in January.6 By June, a Kaiser Family Foundation survey found that 57% of those previously infected got vaccinated.7

As more US employers, local governments, and educational institutions issue vaccine mandates that make no exception for those who have had covid-19,8 questions remain about the science and ethics of treating this group of people as equally vulnerable to the virus—or as equally threatening to those vulnerable to covid-19—and to what extent politics has played a role.

The evidence

“Starting from back in November, we’ve had a lot of really important studies that showed us that memory B cells and memory T cells were forming in response to natural infection,” says Gandhi. Studies are also showing, she says, that these memory cells will respond by producing antibodies to the variants at hand.91011

Gandhi included a list of some 20 references on natural immunity to covid in a long Twitter thread supporting the durability of both vaccine and infection induced immunity.12 “I stopped adding papers to it in December because it was getting so long,” she tells The BMJ.

But the studies kept coming. A National Institutes of Health (NIH) funded study from La Jolla Institute for Immunology found “durable immune responses” in 95% of the 200 participants up to eight months after infection.13 One of the largest studies to date, published in Science in February 2021, found that although antibodies declined over 8 months, memory B cells increased over time, and the half life of memory CD8+ and CD4+ T cells suggests a steady presence.9

Real world data have also been supportive.14 Several studies (in Qatar,15 England,16 Israel,17 and the US18) have found infection rates at equally low levels among people who are fully vaccinated and those who have previously had covid-19. Cleveland Clinic surveyed its more than 50 000 employees to compare four groups based on history of SARS-CoV-2 infection and vaccination status.18 Not one of over 1300 unvaccinated employees who had been previously infected tested positive during the five months of the study. Researchers concluded that that cohort “are unlikely to benefit from covid-19 vaccination.” In Israel, researchers accessed a database of the entire population to compare the efficacy of vaccination with previous infection and found nearly identical numbers. “Our results question the need to vaccinate previously infected individuals,” they concluded.17

As covid cases surged in Israel this summer, the Ministry of Health reported the numbers by immunity status. Between 5 July and 3 August, just 1% of weekly new cases were in people who had previously had covid-19. Given that 6% of the population are previously infected and unvaccinated, “these numbers look very low,” says Dvir Aran, a biomedical data scientist at the Technion–Israel Institute of Technology, who has been analysing Israeli data on vaccine effectiveness and provided weekly ministry reports to The BMJ. While Aran is cautious about drawing definitive conclusions, he acknowledged “the data suggest that the recovered have better protection than people who were vaccinated.”

But as the delta variant and rising case counts have the US on edge, renewed vaccination incentives and mandates apply regardless of infection history.8 To attend Harvard University or a Foo Fighters concert or enter indoor venues in San Francisco and New York City, you need proof of vaccination. The ire being directed at people who are unvaccinated is also indiscriminate—and emanating from America’s highest office. In a recent speech to federal intelligence employees who, along with all federal workers, will be required to get vaccinated or submit to regular testing, President Biden left no room for those questioning the public health necessity or personal benefit of vaccinating people who have had covid-19: “We have a pandemic because of the unvaccinated … So, get vaccinated. If you haven’t, you’re not nearly as smart as I said you were.”

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