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The big questions about Covid-19 booster shots

From the why to the who, we still need a lot of answers about booster shots.

A sign with an arrow points the way to a Covid-19 vaccination clinic.
A sign advertises the availability of Covid-19 booster shots at a vaccination clinic at a San Jose, California, elementary school on November 4.
David Paul Morris/Bloomberg via Getty Images

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.

A patient sits in a chair partitioned off from other chairs by clear screens.
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 customer stands inside a pharmacy near a sign that reads “Limited supply of Covid-19 vaccine, schedule by appointment only.”
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 are low, vaccine protection against severe disease and death is holding up, and especially if few people are dying of the virus, maybe using boosters merely to stop the spread of the disease won’t be worth it.

There is a risk to too much boosting: vaccine side effects, including rare but potentially serious conditions like myocarditis (inflammation of the heart muscle). “Every time you boost your immune response, especially with [Moderna and Pfizer’s] mRNA vaccines, you do have a certain percent of people who are going to get myocarditis,” Offit said. Even with conditions, like myocarditis, that are rare, “if there’s not a clear benefit to that booster dose, that serious adverse reaction becomes more important.”

One possible model, as the virus becomes endemic worldwide, is an annual Covid-19 shot, similar to the flu shot. It’s already possible at American pharmacies to get a flu shot and Covid-19 booster simultaneously, so it’s logistically feasible. This would be a way to keep spread low, helping ensure that the virus doesn’t strike back.

This, however, is all speculative. Experts are in agreement on only one thing here: We need more data.

And then there’s one more question about boosters to answer.

Is the Johnson & Johnson vaccine an exception?

This might seem like a much narrower question than the others, but it’s illustrative.

Among the experts I spoke to, every single one said that the Johnson & Johnson vaccine — originally sold as the one-shot vaccine — likely should have been two shots to begin with. In fact, some experts speculated it would have been two shots if this had been a more typical, less rushed vaccine process, in which Johnson & Johnson had years to study and produce the best vaccine regimen possible.

One major tell is that federal officials approved what they called a “booster shot” for all Johnson & Johnson recipients, instead of limiting the extra dose to specific groups, as they did with other vaccines.

In other words, the “booster shot” for Johnson & Johnson recipients may not be a booster shot at all, but more the equivalent of the second shot that one would get with the Pfizer/BioNTech or Moderna vaccine.

It’s an example of how even the definitions of “booster shot” and “fully vaccinated” remain unclear.

It also shows how hectic times have led to on-the-spot calls riddled with uncertainty. Since the beginning of the pandemic, the public, experts, and officials have made the best calls based on limited and sometimes contradicting information. That includes the rollout of the vaccines and now boosters, up to and including the basic structure of the original vaccine regimens.

This is why all the discussion and debate around boosters can seem so complicated and confusing: It really is complicated and confusing, even for the smartest experts out there. When everyone is working with limited data and hasty judgment calls, the answers aren’t going to be as concrete as anyone would like.

Sometimes that might lead people to move to be safe over sorry, approving boosters that may not have all the evidence for them just yet. “One of the challenges with this pandemic, but really any infectious disease, is you’re making decisions for now, but you’re also trying to anticipate things that could happen,” Jen Kates, director of global health and HIV policy at the Kaiser Family Foundation, told me.

On the flip side, it also means that we, collectively, could be — and probably are — making some wrong calls right now.

As Covid-19 has taught everyone by now, proper responses to pandemics require humility and flexibility. We’re going to need extra doses of both as the world figures out what to do about booster shots.