August 26, 2022 10:15 AM EDT

With nearly all the new COVID-19 infections in the U.S. coming from the Omicron BA.4 and BA.5 subvariants, it makes sense that health officials are considering switching to a different vaccine to protect the public.

White House COVID-19 response coordinator Dr. Ashish Jha expects the first Omicron-specific booster to be available in mid-September at the earliest, if the U.S. Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) authorize and recommend the shot. In late August, both Pfizer-BioNTech and Moderna submitted requests to the FDA for authorization of their Omicron-specific boosters.

But with the fall and winter fast approaching—the seasons when respiratory viruses like SARS-CoV-2 spread even more efficiently, as students return to school and people huddle indoors—getting the booster ready requires a more efficient review and regulatory process. And that includes considering safety and efficacy data from animals, not people.

Back in June, the FDA’s panel of independent vaccine experts met to consider switching the country to a new booster that targets Omicron, given how quickly that variant is dominating new infections. At the time, the two largest COVID-19 vaccine makers, Pfizer-BioNTech and Moderna—which both make mRNA-based vaccines—had developed shots against an earlier Omicron variant, BA.1. The panel decided that if health authorities were going to change the booster shot to target Omicron, the next one should protect against the BA.4 and BA.5 subvariants, which would continue to account for almost all cases in the winter season.

They asked the vaccine manufacturers to develop a new vaccine, one that combined the original vaccine and also targeted Omicron BA.4 and BA.5. At the end of August, both companies submitted data on their new, bivalent vaccines to the FDA for emergency use authorization.

Given the short time they had to develop the shot, however, the data only included information on the safety and efficacy of the booster in animals. Human studies are planned and will be ongoing even if the FDA and CDC decide to authorize the shots and the government starts distributing them. The FDA has also decided to review the animal study data without consulting its advisory committee again.

That has vaccine experts divided. Dr. Paul Offit, a member of the advisory committee, says this strategy makes him “uncomfortable” for several reasons. He notes that the data presented from Pfizer-BioNTech and Moderna in June involving their BA.1 booster shot, which focused on the levels of virus-fighting antibodies the vaccine generated, were underwhelming. “They showed that the neutralizing antibody titers were between 1.5- and two-fold greater against Omicron than levels induced by a booster of the ancestral vaccine,” he says. “I’d like to see clear evidence of dramatic increase in neutralizing antibodies, more dramatic than what we saw against BA.1, before launching a new product. We’re owed at least that.”

While conducting human studies does take more time, Offit says even a small trial involving about 100 people to measure their antibody levels after getting a BA.4/5 booster would be helpful. “You can boost people and measure their neutralizing antibodies two weeks later,” he says. Such information could also be critical in setting realistic expectations for the Omicron booster. The public might feel it’s a panacea that signals the end of the pandemic, but without any data showing how well the booster will protect people from not only getting sick, there might be unrealistic expectations about what the boost can do. “I get a little nervous, frankly, when I hear this [booster] is going to be miraculous,” Offit says.

Other experts see it a little differently. Based on the fact that the mRNA vaccines have been administered to millions of people so far, with relatively few safety concerns, and given that the vaccines have been effective in protecting people from getting hospitalized or dying of COVID-19, even during the latest Omicron surges, they argue that changing the strain of virus in the vaccine doesn’t require the same extensive testing that the original shot did. “The totality of evidence is relevant here,” says Dr. Ofer Levy, director of the precision vaccines program at Boston Children’s Hospital, and also a member of the FDA’s vaccine advisory committee. “We are in a situation where we need to pivot as variants emerge, and if we try to be too rigid in our approach, we will always be behind, and not giving the population optimal protection.”

Levy says that the latest Omicron-specific boosters that the FDA is considering contain a combination of mRNA targets against both the original virus and Omicron BA.4/BA.5, so the data on safety and efficacy from the original vaccine in protecting against hospitalization and death is relevant. While the data on this vaccine does come from animals, using that data to decide whether or not to authorize the booster is a matter of “hedging bets.” There is data showing that even vaccinated and boosted people can get mild to moderate COVID-19 disease, because their vaccine-induced protection is waning, so boosting with a shot that is better matched to the Omicron subvariants circulating now is a reasonable bet, even if the data on its efficacy comes from animals and not people. “I think it’s the right decision,” says Levy.

There’s no guarantee that the FDA will authorize the new bivalent vaccines, although all signs point to an authorization that could come in a week or so. If the shots are released and people get boosted, health officials will be carefully monitoring data from those vaccinees to ensure that the assumptions they made about the safety and efficacy of the booster hold. And hospitalization rates in the coming winter will reveal whether betting on the new Omicron-specific booster was the right decision.

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