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FDA expected to authorize new Covid boosters without data from tests in people

The lack of human data means officials likely won’t know how much better the new shots are — if at all — until the fall booster campaign is well underway.

The updated Covid vaccine boosters, a reformulated version targeting the BA.5 omicron subvariant, could be available around Labor Day. They'll be the first Covid shots distributed without results from human trials. Does that matter?

Because the Biden administration has pushed for a fall booster campaign to begin in September, the mRNA vaccine-makers Pfizer-BioNTech and Moderna have only had time to test the reformulated shots in mice, not people. That means the Food and Drug Administration is relying on the mice trial data — plus human trial results from a similar vaccine that targets the original omicron strain, called BA.1 — to evaluate the new shots, according to a recent tweet from the FDA commissioner, Dr. Robert Califf.

That could be a potentially risky bet, experts say, if the shots don’t work as well as hoped.

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FDA's gamble on the new Covid boosters

Federal health officials hope that the new vaccines will provide stronger protection over the existing booster shots, which still target the original coronavirus strain. But the lack of data in humans means officials likely won’t know how much better the new shots are — if at all — until the fall booster campaign is well underway.

The FDA’s decision to move forward without data from human trials is a gamble, experts say, threatening to further lower public trust in the vaccines should the new boosters not work as intended.

“There’s no reason to think they’ll be unsafe,” said Dr. Celine Gounder, an infectious disease specialist at NYU Langone Health in New York City. “But whether they’ll provide significantly more protection than the original vaccines? Of that I’m skeptical.”

When the FDA authorized the first versions of Pfizer’s and Moderna’s Covid vaccines in late December 2020, it based its decisions on safety and efficacy data from tens of thousands of trial volunteers.

The new shots from Pfizer and Moderna are so-called bivalent vaccines, designed to target the BA.4 and BA.5 omicron subvariants, as well as the original coronavirus strain, in a single dose.

Because of high levels of immunity from prior vaccination and infection, it would be impossible for the companies to test the new boosters in nearly as many people as the original shots.

The FDA asked the drugmakers to update the shots in late June, with the goal of having the shots tested and distributed by the fall. Moderna has finished enrollment for its new booster with 512 participants. Pfizer said it began human testing late this month.

Both companies are expected to release results later this year.

The FDA's decision to consider Covid boosters without human data is in line with how it evaluates modified vaccines for influenza each year. Clinical studies in humans aren't required for the approval of seasonal influenza vaccines, even when they're reformulated for strain changes, said Dr. Jesse Goodman of Georgetown University, a former FDA vaccine chief.

Still, the flu vaccine isn’t a fair comparison, said Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia.

The FDA’s policy on influenza shots is based on decades of experiences with strain changes where the flu vaccines behaved generally in the same way. The U.S. is still on its first iteration of the Covid vaccines, and the mRNA technology has only been in widespread use since late 2020.

The agency is making “huge assumptions” in its consideration of the new Covid boosters, Offit said, adding that it’s possible the new shots may not be any more effective than the existing vaccines.

Not all experts see it the same way.

Dr. Peter Hotez, co-director of the Center for Vaccine Development at Texas Children’s Hospital, said updating the boosters to match the circulating strains makes sense.

“In an ideal world, there is an advantage of combining BA.5 with the original lineage because BA.5 is still with us,” Hotez said. “And if there’s a new variant of concern that emerges later this fall or in the winter, it’s a possibility it’ll look more like BA.5 than the original lineage.”

It's likely that the new Covid boosters will perform better than the existing ones, but uncertainty will remain until trials in humans are completed, Hotez said.

The data in animals is helpful, he said, but there are pitfalls to solely relying on that kind of data.

If you’re 50-plus or you’re 12-plus and immunocompromised and you’re more than three months out from a prior Covid vaccination or infection, you should get a Covid vaccine booster this fall.

Dr. Celine Gounder, NYU Langone health

“The pitfalls are that animals are animals and humans are humans, and although there’s overlap, sometimes there are surprises,” Hotez said. (Children’s Hospital and Baylor College of Medicine developed a low-cost vaccine, dubbed Corbevax, that was cleared for use in India.)

Gounder, of NYU Langone Health, agreed, pointing to a study published this year in the journal Cell Reports that found vaccine-elicited antibody responses in mice can differ from antibody responses seen in nonhuman primates and humans. That study looked at neutralizing antibody responses to the beta and gamma variants, two earlier versions of the virus that spread in the U.S. but never became dominant.

The authors suggested that caution should be exercised when interpreting data obtained from animals. 

But what about data in humans from the BA.1 boosters?

Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston, added that using data from the BA.1 booster may also only provide limited value in determining how effective the BA.5 boosters are.

Barouch, who helped develop Johnson & Johnson's Covid vaccine, noted that that version of the boosters in published data only showed a marginal improvement in immune responses against the original omicron variant compared with the existing boosters.

The clinical benefit of the BA.5 booster compared with the existing vaccines "is not clear," he said.

Who needs a booster?

Despite the uncertainty around the new boosters, most experts agreed that Americans should get whatever vaccine is offered to them this fall and winter. 

Many Americans “need a booster,” Hotez said. 

It’ll be especially important, they said, for people at the highest risk to get the shots.

“Big picture: If you’re 50-plus or you’re 12-plus and immunocompromised and you’re more than three months out from a prior Covid vaccination or infection, you should get a Covid vaccine booster this fall,” Gounder said.

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