New Covid Boosters Were Released Before Human Testing—But Experts Say They're Still Safe

Experts explain the extensive safety history that supported release of new boosters.

Doctor putting a bandaid on a woman's arm after a shot.
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Fast Facts

  • Boosters that target the omicron variant were approved without specific human data on the new vaccine formulation. 
  • The Food and Drug Administration fast-tracked the process, but said the decision was made based on all available evidence, including the safety of previous monovalent COVID vaccines and the booster’s success in mice trials. 
  • Vaccine approval without human clinical trials is not unheard of—in fact, flu shots are developed and updated in this way each year.

The new round of COVID-19 booster shots, reformulated to target the BA.4 and BA.5 omicron subvariants as well as the original strain of COVID, are the first shots to be distributed prior to human trials.

The bivalent COVID-19 boosters —made by manufactures Moderna and by Pfizer, along with its partner BioNTech— were approved by the Food and Drug Administration (FDA) on an expedited basis in order to ensure that they would be released by Labor Day and allow the country to stay one step ahead of of the continually evolving virus and its variants.

Here's what that means and why the FDA took this accelerated approach with the latest round of booster shots.

Why Release of the New Boosters Was Fast-Tracked

The SAR-Cov-2 virus, which causes COVID-19, has been mutating ever since it began spreading years ago, continually evolving into different variants of concern, such as the beta, delta, and omicron variants. Despite this, the United States and other countries continued administering vaccines made for the original virus strain, which doesn't exist anymore.

And while vaccines designed to address the constantly emerging variants have been created, none managed to get through FDA approval before a new variant would essentially replace the strain that the shots were designed to target.

This time around however, manufacturers and the FDA raced to develop and approve a shot targeting the currently the predominant variants, BA.4 and BA.5. To do this, they relied on data from testing the reformulated shots on mice – not on humans.

The FDA issued a statement about its decision to expedite the process, pointing out that it had made its decision based on the "totality of available evidence." The evidence the FDA is referencing includes:

  • Extensive safety and effectiveness data for each of the monovalent mRNA COVID-19 vaccines
  • Safety and immunogenicity data from a clinical study of a bivalent COVID-19 vaccine that contained mRNA from omicron variant BA.1 lineage
  • Non-clinical data obtained using a bivalent COVID-19 vaccine that contained mRNA of the original strain and mRNA in common between the BA.4 and BA.5 strains of the omicron variant

"Based on the data supporting each of these authorizations, the bivalent COVID-19 vaccines are expected to provide increased protection against the currently circulating omicron variant," the FDA said in its statement authorizing release of the new bivalent vaccines, which the agency also refers to as the updated boosters.

Experts Say Updated Boosters Are Safe

Even though the updated boosters were released on an expedited schedule, experts say the shots are safe and that the process parallels a familiar one used to update the seasonal flu vaccine each year. Seasonal flu shots also don't require human clinical trials to be updated and rely upon animal and laboratory testing to develop and evaluate the shot each year.

"I would say this testing strategy really parallels what the testing strategy is for the yearly influenza seasonal shot," said David Martinez, an immunologist at the University of North Carolina at Chapel Hill.

And while the new boosters were approved without specific human data on the new formulation, regulators had all the accumulated human trial data from both the original vaccine development and the development of boosters for variants like BA.1 and beta that were never approved, Deepta Bhattacharya, a professor of immunobiology at the University of Arizona College of Medicine, told Health. Moderna and Pfizer-BioNTech used the same technology to develop the updated bivalent boosters as they did for the original COVID-19 vaccines and the BA.1 bivalent boosters.

Regulators also had years of real-world data at their fingertips on the millions of people who have been infected with the virus, including BA.4 and BA.5.

"I'm not at all concerned about it," said Bhattacharya.

"Now we have a vaccine that matches really well to what's going around," Bhattacharya said. "If we can get people to take the booster, I think it will actually have a pretty big impact on the size of the wave we're facing in the winter."

Larissa Thackray, an associate professor of medicine at Washington University in St. Louis' School of Medicine, also underscored the point that there are minimal differences between this shot and the vaccines that were developed and released for the original COVID-19 strain. And there are even less differences between this update and the one targeting BA.1, which was tested on humans.

"There is nothing special about this spike that has come up in that signature of millions and millions of people infected," said Thackray, referring to the virus' spike protein, which the vaccines are based on.

Importance of Getting Boosters

Research has shown that getting a COVID-19 booster shot can help rejuvenate the waning protection provided by your last shot of the vaccine, especially if it's been many months since you received it or since you had the COVID-19 virus.

For those who are older or immunocompromised, getting a booster shot can be crucial in helping individuals avoid getting seriously sick. At a September 1 meeting, the Centers for Disease Control and Prevention (CDC) recommended that anyone over 12 get the new Pfizer booster, and anyone over 18 get the Moderna booster if it's been a least two months since your last shot.

Martinez said you should also consider waiting if you recently had COVID-19.

The vaccine manufacturers will collect human data curing clinical trials, but it's hard to know for sure when the results will be available, said Martinez. If you choose to wait until human data is available, it could be many months, and the virus could have shifted again by then. By that point, Martinez said, we'll also have real-world data on the people getting the booster, making the trial data largely irrelevant.

Martinez said that getting the new booster is ultimately a personal choice that people should discuss with their doctors. But personally, he said, he will not hesitate to get one.

"I will get a booster 100%," he said.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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