Latest Covid Boosters Are Set to Roll Out Before Human Testing Is Completed

Instead, the agency plans to assess the shots using data from other sources such as research in mice, the profiles of the original vaccines and the performance of earlier iterations of boosters targeting older forms of Omicron.

“Real world evidence from the current mRNA Covid-19 vaccines, which have been administered to millions of individuals, show us that the vaccines are safe,” FDA Commissioner

Robert Califf

said in a recent tweet. The FDA pointed to Dr. Califf’s tweets when asked for comment.

Clearance of the doses, without data from human testing known as clinical trials, is similar to the approach the FDA takes with flu shots, which are updated annually to keep up with mutating flu viruses.

The approach has raised concerns, however, among some vaccine experts who have urged the agency to wait.

“I’m uncomfortable that we would move forward—that we would give millions or tens of millions of doses to people—based on mouse data,” said

Paul Offit,

an FDA adviser and director of the Vaccine Education Center at Children’s Hospital of Philadelphia.

The comparison with flu vaccines isn’t sound, Dr. Offit said, because flu viruses mutate so rapidly that shots from one year don’t offer protection for the next, while currently available Covid-19 shots continue to keep people out of the hospital.

In addition to evaluating the boosters without clinical-trial data, the FDA won’t convene another element from its earlier Covid-19 vaccine reviews: a meeting of advisers who make recommendations whether the agency should authorize a shot.

The FDA scrapped the meeting, Dr. Califf said in his tweets on the subject, because the committee discussed the matter in June, and the agency doesn’t have new questions warranting its input.

The Covid-19 vaccines available in the U.S., which were first authorized for use in December 2020, haven’t been modified until now, though the virus they were designed to target has evolved.

The shots held up well against earlier strains, researchers found, but weren’t as effective against the newest Omicron subvariants like BA.5.

In planning for a fall booster campaign, federal health authorities in late June directed

Pfizer Inc.

and its partner

BioNTech SE,

and

Moderna Inc.

to update their shots to target BA.5, an Omicron subvariant called BA.4 and the original strain of the virus.

“We’ve validated the process several times over and continue to produce safe and effective vaccines against Covid-19,” a Pfizer spokeswoman said. Moderna said all current data indicates its shots are safe and effective.

Human trials for Moderna’s vaccine targeting the subvariants have started, and for the Pfizer-BioNTech vaccine are expected to start this month, the companies have said. Results won’t be available, however, before the U.S. government’s planned fall booster campaign.

“If we waited for clinical-trial results, thank you very much, we’d get them in the spring. It takes time to do clinical trials,” said

William Schaffner,

professor of medicine at Vanderbilt University Medical Center and a nonvoting liaison to the Centers for Disease Control and Prevention committee that will decide whether to recommend the shots, should the FDA sign off. “This is just an updating of the previous vaccine that we used.”

The retooled shots are similar to the original shots, but customized to fight the latest variants, much like keys that are nearly identical but have slightly different ridges and valleys, said

John Grabenstein,

director of scientific communications for Immunize.org, a nonprofit that seeks to boost immunization rates.

The similarities make it very reasonable for regulators to weigh the overwhelmingly safe track record of the original series when considering the new shots, he said.

The FDA has reviewed test results from a shot that Moderna modified to target an early version of Omicron as well as the ancestral strain of the coronavirus. The study found the shot generated a significant amount of antibodies in humans compared with the company’s currently available booster shot. That shot is now approved in the U.K.

The agency also looked at human data from Pfizer and BioNTech finding that their experimental shots, updated to target an earlier form of Omicron, also boosted antibody levels significantly. The companies have submitted one of those shots to the U.K., EU and Canada for authorization, Pfizer has said.

Such findings give the FDA confidence that the newest modified shots will also work well, said a person familiar with the agency’s deliberations.

“As we know from prior experience, strain changes can be made without affecting safety,” Dr. Califf said in a tweet.

Dr. Offit, however, said he would like to wait for clinical-trial data showing the shots are effective before asking people to take them.

“If you have some evidence that this is likely to be of value, sure,” he said. “But if you don’t have evidence, and you know that the current vaccine does offer protection against severe disease, I don’t think it’s fair to ask people to take risks.”

—Jared S. Hopkins contributed to this article.

Write to Liz Essley Whyte at liz.whyte@wsj.com