Covid-19 booster shots: what we know about immunity, availability, and eligibility

All of a sudden, it seems a lot of people are getting Covid-19 booster shots. The additional doses of Moderna’s and Pfizer/BioNTech’s vaccines have been approved in the US for people 65 and up as well as at-risk populations, such as people with certain health conditions and front-line workers. Everyone who got the Johnson & Johnson vaccine can now get a booster. Meanwhile, Pfizer is asking for approval of booster shots for all Americans who previously got its vaccine.

But underlying the booster mania is a question that many experts say remains unanswered: What exactly is the point of the extra shot?

From an individual perspective, the answer seems straightforward. The boosters appear to reduce the chance of infection and, likely, transmission of the disease, giving the immune system a refresher. So getting a booster could protect you and those around you by making you less likely to get infected in the first place.

But this simple answer invites a new set of questions. If the desired outcome is maximum protection from any kind of Covid-19 infection, does that mean people should get vaccinated every three, six, nine, or 12 months as antibodies continue to fade? Is that even practical? And does society as a whole benefit from vaccinated people getting even more protection, especially if doses could instead inoculate unvaccinated people around the world?

Some experts say there’s not enough clarity around these questions, even as the country embraces a widespread booster strategy. “What is it that you’re trying to achieve?” Céline Gounder, an epidemiologist at New York University who’s advised President Joe Biden, told me. “That needs to be clearly defined.”

But the Biden administration, which has promised and encouraged booster shots for months, is facing an uncomfortable reality: A campaign for boosters might be the best thing they can do at this stage of the pandemic, especially given who’s most likely to heed guidance from federal officials.

Every expert will say that immunizing unvaccinated people is a far more effective means of controlling Covid-19 than giving a booster to those who are vaccinated. But, after the better part of a year, only about 70 percent of US residents 18 and older are fully vaccinated. Roughly one in five adults remain adamant that they will definitely not get vaccinated or will only do so if it’s required. Months into campaigns pushing people to get vaccinated, those who are still unvaccinated just seem very difficult to move to the vaccinated side.

People who have already gotten vaccinated are probably more persuadable; after all, they already got their first shots. For the Biden administration, then, pushing these people to get another shot may be the easiest path to boosting population-level immunity to some extent, even if it would be preferable to get unvaccinated people to get a shot or two instead. It’s a pragmatic call about what can be done now, rather than aiming for the ideal.

Still, it’s clear that there’s a lot of uncertainty about the why, who, and when of booster shots. Even experts who closely follow this topic for a living don’t have all the answers to basic questions, from whether waning immunity is a big problem to what the definition of a “booster shot” is. When I asked Brown University School of Public Health dean Ashish Jha about unanswered questions, he responded, “Well, there’s a lot.”

These unanswered questions make it harder to determine what booster shots are supposed to do — and whether their benefits truly outweigh the costs.

Is waning immunity really a big problem?

The big argument for booster shots is that the protection from vaccines appears to wane to some extent. But even here, there’s a lot we don’t know.

Some evidence suggests that vaccine-induced immunity against any Covid-19 infection, including those that lead to no illness or mild symptoms, does wane. An earlier study from the CDC found that vaccine effectiveness against infection among New York adults fell from 92 percent to 75 percent between May and July. A more recent study in Science, looking at Veterans Health Administration data, found vaccine effectiveness against infections among military veterans declined from 88 percent to 48 percent between February and October.

Vaccines still offer some protection against infection; 75 percent or 48 percent is better than zero percent. Separate studies, from the Netherlands and the UK, have also shown that the vaccines continue to reduce the risk that someone will transmit the virus — not to zero, but by a statistically significant extent.

And vaccine-induced protection against severe disease and death has mostly held up. In the New York study, vaccine effectiveness against hospitalization was more than 93 percent. In the veterans study, vaccine protection against death was 82 percent for veterans younger than 65, and 72 percent for those 65 and older. Other CDC data, going into September, found unvaccinated people are 11.3 times as likely to die from Covid-19 as those who are fully vaccinated.

Army veteran William Craig waits to see if he has a reaction after receiving both a Covid-19 booster shot and an influenza vaccine at the Edward Hines Jr. VA Hospital in Hines, Illinois, in September.

Scott Olson/Getty Images

We actually don’t know how much these numbers are measuring the vaccines’ protections waning versus the delta variant better evading immunity. And there are statistical paradoxes that can further complicate all of this data.

Still, experts say there is good reason to expect that protection against any infection was always going to wane: Antibodies fade over time. That’s normal; it’s just what the immune system does. But if the immune system keeps some defenses around and can kick into gear if an infection does occur, then someone might get sick and maybe even spread the virus but be largely protected from hospitalization and death.

“When people come into the hospital, they’re not in the ICU because they’ve not gotten a third dose,” Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, told me. Typically, “they’re in the ICU because they haven’t gotten any doses.”

Waning immunity against infection may just be part of the new normal. Many experts now believe Covid-19 will become endemic, sticking around in a similar form like, say, cold and flu viruses. But in that scenario, the virus will very likely be defanged by a mix of natural and vaccine-induced immunity, as well as better medications and other treatments.

“I’m still seeing on Twitter the desire for elimination, but it’s not going to happen,” Monica Gandhi, a doctor and an infectious disease expert at UC San Francisco, told me. Still, she argued, “We’re so lucky. We prevented the worst thing that could ever happen, which is to get super sick from a horrible new virus.”

In that view, maybe waning immunity against infection isn’t, in the long run, that big of a deal.

But we aren’t there yet. Much of the population still isn’t vaccinated, and more than 1,200 Americans are still dying of Covid-19 each day. Right now, there’s too much spread and too much death, so we need all the immunity we can get to fight off the virus.

That means booster shots could ultimately be situational. They might make sense, especially for vulnerable people, when outbreaks are bad or likely to get bad in the future (such as in the fall and winter), but we wouldn’t be trapped in a never-ending cycle in which people should get shots every few months for the rest of their lives.

That gets to another question.

Who actually needs booster shots?

There are two groups that experts widely agree benefit from booster shots: the elderly (65 and up) and the immunocompromised.

For older adults, there are two major considerations. One, this group has always been much more likely to die of Covid-19: People 65 and up comprise 75 percent of Covid-19 deaths in the US. Two, older immune systems seem to get less from the vaccines or, at the very least, see protection wane more quickly; as the study of veterans found, vaccine effectiveness against death was about 10 percentage points lower for those 65 and older than those younger than 65.

People who are immunocompromised, meanwhile, may not get much if any benefit from just the two shots of Pfizer/BioNTech’s or Moderna’s vaccine or the one shot of Johnson & Johnson’s. But there’s some evidence an additional shot or more may increase levels of protection.

Beyond those two groups, Gandhi said, “there’s just not great evidence for boosting.”

There’s even a case that no one should get a booster shot as long as so much of the global population is unvaccinated. Supplies — as well as attention and other resources involved in a vaccination campaign — are limited. Every booster shot that goes to a previously vaccinated American could be one that goes to an unvaccinated person in the Global South, particularly in Africa, which currently has vaccination rates below 10 percent. As long as the coronavirus spreads unchecked globally, it’s more likely to transform into a worse variant — one that’s deadlier, more contagious, or evades current immunity.

“It’s not just about morality and ethics,” Gounder said. “We are so losing sight of what is most important here.”

Still, chances are that at least some people will need boosters after global vaccination efforts scale up. And some people are getting boosters regardless in the meantime. For now, there’s consensus that older and immunocompromised groups should be at the front of the line. But for everyone else, there’s a lot more skepticism — out of a concern that the extra doses for vaccinated people just don’t come out on top of a cost-benefit analysis.

Okay, but should you, the reader, get a booster shot? Most experts have told me that if you’re eligible, you should.

For one, it’s better to be safe than sorry with your personal health. And while it might be better for the dose you’re taking to go elsewhere, that shot’s fate is already sealed — it’s already purchased by the federal government for domestic use and allocated to your local pharmacy, doctor’s office, or wherever else you’re going to get a vaccine. This is a problem to be solved upstream by policymakers, not individuals with personal boycotts of booster shots.

But then there’s another problem: the question of when to get a booster.

A man checks in for a Covid-19 booster shot at a grocery store pharmacy in downtown Denver on November 3.

David Zalubowski/AP

How often will people need booster shots?

The simple truth: No one knows how often booster shots will be necessary.

“What are the long-term benefits of a booster? How long does it last? Will we need another booster a year from now? Will we need another booster sooner than that?” Jha said. “We don’t know the answers to that.”

The recent evidence suggests vaccine-induced protection against infection starts to wane after a few months. But that’s only after a person has been fully vaccinated for the first time.

There’s some reason to hope that a booster shot could produce a more permanent effect. “There’s some reasons to believe, immunologically, that once you get a booster six months after your second shot, that that should have a lot more durability than the first two shots did,” Jha said. But, right now, we “can’t prove that. We don’t know for sure. We don’t have that long-term data.”

Even if vaccine-induced immunity does wane, there are other considerations. If in six, eight, or 12 months, Covid-19 cases…

Covid-19 booster shots: what we know about immunity, availability, and eligibility