Four Big Questions About the New Covid-19 Boosters, Answered

Covid-19 Bivalent Booster Shot

A pharmacist prepares to administer a Covid-19 booster shot to help protect a patient against the Omicron subvariants BA.4 and BA.5.
Scott Olson / Getty Images

Every day in the United States right now, a jumbo-jet full of patients dies from Covid-19. To be more precise, 314 individuals have died every 24 hours over the past week, while 70,000 got infected and 4600 were hospitalized. Even though the Biden Administration predicted in May that 100 million Americans would be infected with the coronavirus this fall and winter, much of the public has put the pandemic in the rear-view mirror in their pursuit for normalcy.

That approach carries some risk. “The vast majority of the United States has had Covid at some time, but that is not going to protect you for the rest of your life,” says Katherine Poehling, a pediatrician and epidemiologist at Wake Forest University School of Medicine. “Our immunity is going to wane because it does to all coronaviruses.”

Given this diminishing protection, booster shots have become a much-touted public health tool. Nearly half of fully vaccinated Americans, or 109 million individuals, have received at least one booster since the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) first recommended the extra doses in August 2021.

But the viral landscape looks remarkably different this year. In June, Omicron subvariants BA.4 and BA.5 became the chief drivers of the pandemic and now constitute approximately 90 percent of the U.S. Covid-19 caseload. The greater transmissibility of these mutated strains also means that previous vaccines and boosters are less effective at protecting the body against the virus.

On September 1, the CDC unveiled its recommendations for updated boosters to help fight the new variants. Dana Hawkinson, an infectious disease physician and the medical director of the Infection Control and Prevention program at the University of Kansas, describes these boosters as evoking a little bit more protection against infection and “maximal protection” against severe disease, hospitalization and death.

With the ongoing rollout of these updated boosters, the public understandably has a lot of questions. So, we’ve reached out to medical experts for the answers you need to know.

What is different about these boosters, and how will they help against Covid-19?

Just like the earlier vaccines, the most recent boosters employ mRNA technology to train your immune system in the event of an actual coronavirus infection. The key difference, says Hawkinson, is that the updated boosters are “bivalent.” In other words, half of the shot is designed to protect against the original vaccine strain and the other half to protect against the BA.4 and BA.5 versions.

The bivalent boosters achieve this protection by enabling the body to create two types of spike proteins, which imitate components used by the original virus and BA.4 and BA.5 strains. That in turn primes the immune system to identify and destroy the virus, essentially bolstering its response to different possible variants.

Moderna and Pfizer had previously worked on bivalent boosters that showed promise against targeting BA.1, a closely related Omicron subvariant. “But then, BA.4 and BA.5 really jumped on the scene this summer,” says Poehling, “so the FDA said, ‘Let’s switch to BA.4 and BA.5 for our booster vaccine.’” This shift was prudent. Unlike the other Omicron subvariants, BA.1 will probably not be involved in the anticipated winter surge.

As the bivalent boosters are widely deployed, researchers will be watching carefully to see how they hold up against the original “monovalent” version. One meta-analysis available in preprint (meaning the data are not yet peer-reviewed) suggests that vaccines adjusted to target specific variants can provide broader protection. However, the study’s findings also raise questions about why the bivalent boosters protect against the original vaccine strain as well, given that the Omicron variants have overtaken the pandemic. One possible explanation, offered in Science, is that a future variant could bear more resemblance to an earlier strain. In this case, the updated boosters may be able to act as something of a catch-all.

I have read that the updated boosters have been tested only in mice. Should I be concerned?

Because of an anticipated surge in Covid-19 cases this fall and winter, the FDA authorized the boosters based on mouse studies rather than human clinical trials, which take much longer to conduct and would not have offered results in time. Correspondingly, some experts are concerned about extrapolating from animal studies to a population-wide rollout, raising a number of questions given the absence of human data. How will the boosters work in individuals who already got four or five doses of the vaccine? What level and durability of protection will these boosters provide? “That’s really the crux of what we’re trying to figure out,” Hawkinson says, because no data are available for how these particular boosters will impact human immune systems.

Given that most fully vaccinated individuals haven’t even received the old boosters, some experts also worry that the mouse trial narrative will further exacerbate booster hesitancy. But Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical advisor to the President, told The New York Times, “I take great issue with those who say, ‘Oh, you’re just approving this with mouse data.’ We’re authorizing this with the totality of the evidence that we have.”

That evidence includes human clinical trial data from the earlier BA.1 bivalent boosters and empirical safety data from the millions of individuals who have gotten Covid-19 vaccines. As FDA Commissioner Robert Califf recently tweeted, “as we know from prior experience, strain changes can be made without affecting safety.” In other words, given the old recipe of mRNA technology and a bivalent formulation, the ingredient substitution to BA.4 and BA.5 doesn’t require the same level of testing.

Although these boosters mark the first time Covid-19 shots have been authorized without human testing, according to Poehling, this is the regular approach for the flu vaccine: an updated version is released annually without first undergoing new clinical trials. And at a recent White House briefing, Fauci said the U.S. is likely moving toward an influenza-style vaccination program for Covid-19, with annual shots tailored to protect against the dominant variants.

None of this is to say that human studies aren’t important or helpful. Although Hawkinson emphasizes the need for efficiency during a rapidly moving pandemic, he admits, “anytime you’re giving something to humans, you want it tested in humans.” So, Pfizer and Moderna have begun human clinical trials for their updated boosters, with results anticipated later this year. But Poehling says a delay in approval would have been disastrous, as statistical models indicate that it would have caused 9700 deaths and 137,000 hospitalizations.

How important is it that I get one of these new boosters?

With winter rapidly approaching, SARS-CoV-2 and other respiratory viruses will spread more efficiently given that colder weather impairs our immune systems and forces individuals to spend more time indoors. The situation might be further exacerbated by people returning to school and the workplace while states continue to relax Covid-19 restrictions. Poehling points to Australia’s early, vigorous flu season, one of its worst in several years, as a warning sign for impending surges in flu and Covid-19 cases in the U.S. A greater risk of infection, along with reduced vaccine protection, are two key reasons to get one of these boosters.

“You do get a benefit of decreased chance of infection even from the original vaccines,” Hawkinson says, “but that benefit is minimal. And we saw that it was even more minimal as we’ve gotten to the new variants.” On top of the BA.4 and BA.5 variants being more immune-evasive, vaccine-induced immunity naturally wanes over time as an individual’s initial surge in antibodies fades away.

The hope is that an updated booster will help restore an individual’s immunity and provide greater protection against future Covid-19 variants. The FDA is so confident in the superiority of the bivalent boosters that monovalent boosters are no longer authorized for most individuals.

But not everyone will derive the same level of benefits from these boosters. Some experts are skeptical of the shot’s value in younger, healthier individuals, and Poehling acknowledges that the groups at highest risk—older adults, nursing home residents, those with chronic medical conditions—are the most in need. But she still thinks that everyone should seriously consider getting a booster because it will “keep us out of the hospital and keep us from dying,” offering a fresh coat of protection against a relentless virus. “It’s important for all of us.”

Who is eligible right now, and when should I get my booster?

Individuals age 18 years or older can get the Moderna booster, while those age 12 years or older can get the Pfizer booster, per CDC guidelines. Although the CDC does not recommend mixing and matching vaccines in the primary series (i.e., dose one and two), fully vaccinated individuals can get either one of the two boosters, Hawkinson says. Importantly, no bivalent booster has been authorized for children aged 5 through 11, but they can still receive the original monovalent one. Children aged 6 months to 5 years old can receive a two- or three-dose vaccine regimen, but boosters of any kind have yet to be approved.

Having already begun to be shipped to pharmacies, community health centers and hospitals across the country, the updated boosters will become increasingly available in the coming days and weeks. With a search filter for “newly authorized bivalent” boosters, can help individuals determine availability and book an appointment close to them.

Ultimately, however, the decision of when to get one of the boosters will vary from person to person. For instance, if an individual gets the monkeypox vaccine, the CDC recommends waiting four weeks before getting one of the Covid-19 boosters. (The flu vaccine, by contrast, has no recommended waiting period and can be given simultaneously.) Similarly, the CDC recommends waiting at least two months after receiving a Covid-19 dose and three months after a Covid-19 infection before getting one of the bivalent boosters. This waiting period may help maximize the booster’s benefits while also reducing the risk of extremely rare side effects, such as myocarditis, according to Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research.

For healthy adults who have recently been infected or vaccinated, some experts say they can wait a bit longer to get their boosters, as long as six months when antibody levels seem to plateau, according to a recent study in the Journal of the American Medical Association. Some individuals might also try to time their boosters so that they can maximize immunity before they enter the holiday season or as case counts begin to rise. These decisions should be made with care and ideally in consultation with a physician, Hawkinson says, because booster protection is not instantaneous and there can be a real risk to miscalculation. So, for individuals who haven’t recently been infected or vaccinated, it’s probably wise to get boosted sooner rather than later.

The bottom-line is that the new Covid-19 boosters may offer an opportunity for the U.S. to get ahead of the virus, as opposed to always playing catch-up. Poehling says, “getting on top of this and giving people protection to what’s circulating seem to be the most prudent approach.”