The word booster kicked off the pandemic benign and simple, a chipper concept most people linked to things such as morale and rockets. Then, at the start of 2021, the word began to undergo a renaissance. By summer’s end, booster was a common fixture of headlines and Twitter trends; it was suddenly tethered tightly to words such as shot, vaccine, and immunity online, as experts and nonexperts alike clamored for the more, more, more promise of extra protection against SARS-CoV-2. According to Elena Semino, a linguist at Lancaster University, in the United Kingdom, English-language news reports now deploy the word booster about 20 times more often than they did in pre-COVID times.

The pandemic has, in effect, boosted boosters into the public sphere. And yet, we are still really bad at talking about them. In the top echelons of the CDC, in the back alleys of Twitter, no one can seem to agree on who needs boosters, or when or why, or what that term truly, technically means—even as additional shots that officials are calling boosters continue to enter arms. Some experts insist that boosters are necessary; others vehemently disagree; a few have insisted that we shouldn’t be using the B-word at all. Discussions among the rest of us have been no less chaotic. A September poll from the Kaiser Family Foundation shows that more than a third of respondents find information on boosters to be confusing instead of helpful. Last week, my own mother, a retired medical technologist, asked me whether she should get a booster. “What do you think the booster is for?” I asked her. She paused. “Well,” she said, “I don’t know.”

The battle over boosters is about more than semantic precision. Without properly defining what these additional injections are, and what they’re intended to accomplish, experts can’t demarcate success. Defining the goals of boosters now would help us figure out who needs them now, who might need them eventually, and even how often we’ll all need them in the future, if we need them at all. To fully capture what boosters can and should do, though, we may need to reframe what that word means to us—or, as some have argued, dispense with it entirely.


Booster isn’t new to the vaccine lexicon; American adults, for instance, are asked to tangle with the term every 10 years or so to maintain their defenses against tetanus. But the word sprouted independent of immunization, as the linguist Ben Zimmer recently wrote. Its roots date back to 1801 at the latest, though it’s hard to pinpoint when, or from where, it actually arose. The term has since gained a pretty straightforward connotation—“upward movement.” A boost is a lift, a push, an increase, the ability to take us “to new heights, further than we could otherwise go,” Neil Lewis Jr., a communications and social-behavior expert at Cornell University, told me.

We use boosters to raise up children sitting in cars, and to launch rockets into the beyond; boosters naturally evoke ideas of support or benefit, which makes them a PR windfall. By the 1940s or so, perhaps earlier, booster had entered the immunizer’s lexicon, and might have made additional doses of tetanus, diphtheria, pertussis, and polio vaccines more palatable to the public. It almost certainly helped “put a positive spin on the need for extra shots” of the inactivated polio vaccine in the latter half of the 20th century, Elena Conis, a vaccine historian at UC Berkeley, told me.

But this perky portrait of boosters might obscure why we need them at all. There’s more than one reason to administer an additional dose of the same vaccine. Many immunologists and vaccinologists draw a distinction between doses in the primary series, which create immune protection in a person who’s never been inoculated before, and boosters, which replace those defenses when they’ve started to fade. The primary series can comprise a single dose, or more commonly, multiple, as with two-dose MMR shots, or three-dose hepatitis B vaccines. The aim of a primary series is to reach and maintain a protective threshold, with each dose building iteratively on the quantity, quality, and durability of that defense, and a person can’t be considered fully vaccinated without finishing those initial shots. But once they do, they might never need another injection again. Primary-series doses, in other words, are generative.

Boosters are the optional second chapter in this story. They’re not necessary for all vaccines —just the ones whose protection appears to ebb, usually over the course of years, à la the once-per-decade tetanus touch-up. Boosters are restorative, meant to put back something that was once there, but has since been at least partially lost. An added shot “gets you back up to some threshold we know is important,” Rishi Goel, an immunologist at the University of Pennsylvania, told me. (Not every shot administered in regular intervals is a booster: The annual flu shot, whose ingredients change every year, is issued less because our bodies are forgetting a specific strain, and more because the many viruses we encounter change so rapidly.)

What we now refer to as boosters, then, might be better described as refresher, refill, or reminder shots—something that signals not just growth, but growth from a place of temporary loss. This mirrors the way several Romance languages describe booster shots: Spanish speakers say refuerzo, a term that signifies reinforcement, while Italians say richiamo, and the French say rappel—both words that signify recollection.

For COVID-19 vaccines, booster is already a popular term, but it’s not obvious how restorative the additional shots are, in terms of guarding against the coronavirus. In one group, at least, third shots are generative: people who are moderately or severely immunocompromised, and may not have marshaled a sufficient immune response to their initial vaccine doses. “In this population, that’s really clear,” Grace Lee, a pediatrician at Stanford University and the chair of the CDC’s Advisory Committee on Immunization Practices, told me. (There is still, frustratingly, a huge paucity of data on the one-dose Johnson & Johnson vaccine, though several experts have told me in recent weeks that J&J’s regimen may become a two-shot primary series for everyone, based on the company’s recent findings.)

When it comes to the rest of us, especially people who are younger and healthier, experts remain divided on how to categorize third shots. Anthony Fauci told me recently that he’s very much in the generative camp: “I bet you any amount of whatever that when we finally look back on it,” he said, three doses is going to be “the standard regimen for an mRNA vaccine.” (Still, even Fauci’s been blurring the semantic boundaries. In a recent interview with my colleague Ed Yong at The Atlantic Festival, he alternately described the shot as a “third dose,” a “third-shot booster,” and a “third booster shot” in a five-minute span.)

If that turns out to be the case, experts would first need to show that what the first two doses gave us wasn’t good enough, opening up the opportunity for a third jab to make our defenses “more durable, and much more able to protect us” than they were with two shots alone, Paul Offit, a vaccine expert at the Children’s Hospital of Philadelphia, told me. But so far, there’s really no clear evidence to suggest that a third shot elevates us into a new tier of protection, especially against the worst COVID-19 outcomes. The two-dose mRNA vaccines are still blocking hospitalizations and deaths to an extraordinary degree. “If the goal is to prevent serious illness, it does that,” said Offit, a member of the FDA’s vaccine advisory committee. Data from Goel and others back this up on a molecular level. Even several months after getting their second primary doses, vaccinated people (with the possible exception of some folks who are older or not in great health) appear to retain massive legions of immune cells that remember SARS-CoV-2 well enough to thwart it. Some of these defensive populations even seem to be refining themselves into larger and more sophisticated pools of assassins over time, long after the vaccine itself is gone.

So maybe these third injections are restorative, meant to replace a defense that has withered over time. The burden of proof for that would be twofold: identifying some sort of waning, as well as evidence that an extra shot reverses the ebb. Inklings of the former have, arguably, started to appear. Vaccines still reduce the chances of getting infected; experiencing nasty, lingering symptoms; and passing the virus on to others. But since the spring, mild-to-moderate sicknesses have become a bit more common among the inoculated. Though some of that’s definitely attributable to the rise of the super-contagious Delta variant, this trend also likely reflects the decline in antibody levels that happens after all vaccinations, as the body, freshly roused by the shot’s contents, starts to return to a peacetime state.

That leaves the actual restoration bit. In recent presentations to expert committees that advise the FDA and CDC, Pfizer executives crowed about sky-high antibody levels appearing after vaccine recipients got a third shot—evidence, they said, that the injections were bringing the body’s frontline defenses back up to snuff. That could make it easier for people to fight off infections early, before they turn symptomatic, or spread to someone else.

But again, antibody levels always drop. (If the body kept pumping out antibodies ad infinitum, it would drive itself into the ground—and rapidly thicken its own circulatory system into a protein-packed sludge.) That raises the possibility that post-booster bumps in protection, too, might be only temporary. “That’s where I get tripped up,” Stanford’s Lee told me. “If we’re boosting to boost antibodies, will we need another dose six months from now?” Some researchers (and Pfizer’s CEO) think we might need annual, even twice-annual, COVID shots for as long as the virus is with us. That prospect can feel demoralizing, and experts worry about the message it sends to the unvaccinated. “I hear the skepticism,” Lewis, of Cornell, said. “‘Well, if this stuff is just going to keep fading away, what’s the point?’”


Another sector of the population doesn’t mind the threat of repetitive boosting—“the more protection, the better” has become a common refrain, as some seek out fourth, fifth, even sixth shots. Cloaked in this behavior is another downside of using booster as our linguistic crutch: its near-unilateral promise of more and more benefit, as if shots can be stockpiled like so many rolls of toilet paper. Some Americans have clearly been clamoring for spare shots since at least the spring, among them booster bandits who wriggled through loopholes to nab their jabs ahead of schedule. “With boosters, you’re getting more, and as consumers, we like more,” Stacy Wood, a marketing expert who studies public perception of vaccines at North Carolina State University, told me. It’s a natural response in times of crisis, she said, to “buffer against a lack of future supply.”

Vaccines, unfortunately, don’t work like that. Boosting too early and too often can be counterproductive, for the same reasons that cramming the night before a big exam is: Immune cells, being the students of microbiology that they are, can’t internalize all that information at once; there’s little point in foisting a second lesson on them when they’re still frantically trying to take notes on the first. Immune responses also have ceilings, and administering shot after shot after shot, even somewhat spaced out, could eventually drag the body toward the point of diminishing returns. “That’s a waste of a vaccine,” Lauren Rodda, an immunologist at the University of Washington, told me. After about half a dozen tetanus boosters, for example, “no matter how many more you give, you can’t get any higher antibody response,” Mark Slifka, a vaccinologist at Oregon Health & Science University, told me. We actually used to boost more often against tetanus. But countries loosened their requirements after realizing there was no point. Shots also come with side effects, including a small number that, though quite rare, can be dangerous, Slifka said. Data on the safety of third COVID-19 shots are still being gathered, and although the expectation is that they should be very well tolerated, all this is uncharted territory.

Such complex calculus is tough to encapsulate with a term like booster. This, perhaps, is part of the fallout when technical, hyper-specific terms “leak into other communities,” Semino, the linguist, told me. “All of a sudden, something for a professional community is being used for everyone.” Pre-pandemic, most of us didn’t automatically tie boost to vaccines. Now we’re being asked to. And it’s very difficult to know how much our booster preconceptions are coloring our attitudes around extra shots—when to get them, how often to get them, when to stop.

Calling them reminder shots—vaccines that offer a richiamo or rappel—skirts some of those issues, capturing dimensions of immunity that booster does not: that there is loss; that there is, sometimes, a replenishing; that protection is not linear, and can shift up or down over time. This framing could also be a more clear-eyed way to assess global equity. Boosters, by default, top off resources that have already been given. If the goal is truly to tamp down transmission, infection, and disease on a wide scale, generative shots—especially first doses—will go much further than restorative ones. “Public health is a collective phenomenon,” Martha Lincoln, a medical anthropologist at San Francisco State University, told me. “We can’t pass the buck to individual immune systems.”

Boosting and primary-dosing aren’t mutually exclusive goals. But they draw resources from the same, finite pool. And Lee worries that our third-dose mania might be a bit myopic, especially with so many still unvaccinated here in the United States, and around the globe. “In a highly vaccinated population, boosters can really put you over the edge, and reduce overall circulation,” she told me. Eventually, that will be a priority—tailoring our vaccine rollouts to ensure that we’re cutting down on all kinds of infections, to the extent that we can. Right now, though, with Delta still erupting throughout unimmunized communities, and the health-care system unbearably overstretched in many parts of the country, “we’re not even close to where boosters are going to do anything [other] than provide some individual level benefit.” Our own bodies, after all, seem to be remembering SARS-CoV-2 just fine. It’s everyone else we can’t afford to forget.



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