Was New COVID Vaccine Booster Tested on Only 8 Mice? What We Know

New COVID boosters that will be available by week's end are receiving some scrutiny due to clinical trials being performed on mice and not humans.

The boosters made by Pfizer-BioNTech and Moderna were approved by the Food and Drug Administration (FDA) on Monday. On Tuesday, a panel of advisers to the Centers for Disease Control and Prevention (CDC) voted 13-1 in favor of its broad use for individuals 6 months and older due to fears of increased respiratory illnesses nationwide this fall and winter.

The new vaccines will be available at national pharmacies including CVS and Walgreens, and at local pharmacies, doctors' offices and public health departments based on location.

These updated boosters will be the first released to the public without any humans involved in the clinical trials, raising questions from some about the efficacy.

In June, the FDA requested manufacturers to develop a new booster for the fall season to target the then-dominant XBB.1.5 subvariant. The new bivalent shots fight the BA.5 omicron subvariant.

An FDA spokesperson declined to comment to Newsweek specifically on the testing mechanisms, adding that Pfizer and Moderna could speak more about their protocols.

The spokesperson alluded to a press release issued by the FDA, which states that the updated mRNA vaccines are manufactured using a similar process as previous formulations.

"In studies that have been recently conducted, the extent of neutralization observed by the updated vaccines against currently circulating viral variants causing COVID-19, including EG.5 and BA.2.86, appears to be of a similar magnitude to the extent of neutralization observed with prior versions of the vaccines against corresponding prior variants against which they had been developed to provide protection," the release says.

"This suggests that the vaccines are a good match for protecting against the currently circulating COVID-19 variants."

It adds that "the benefit-risk profile is favorable" to those six months and older who are encouraged to get these boosters, based on previously authorized and approved vaccines.

CDC Director Mandy Cohen said in a statement that "We have more tools than ever to prevent the worst outcomes from COVID-19."

Cohen, in a New York Times op-ed published Wednesday, acknowledged that coronavirus will likely always be with us, but that serious illnesses, hospitalizations, deaths and long-term effects can and should subside with the new science—which also can help defend against respiratory syncytial virus (RSV) and flu viruses.

"Some viruses, however, change over time," she said. "This coronavirus is one of them. It finds ways to evade our immune systems by constantly evolving. That's why our vaccines need to be updated to match the changed virus. Even though many Americans have been exposed to previous versions of the virus because they've been infected, that protection decreases over time."

More than 670 million doses have been administered in the U.S. since 2021.

Dr. David Wohl, an infectious disease expert at the University of North Carolina School of Medicine, told Newsweek via email that tremendous attention has been paid to monitoring the safety of the COVID-19 vaccines, including in children.

"With so many people across the world having been vaccinated, often repeatedly, there are large data sets that allow for detection of safety signals," Wohl said. "We saw how with the Johnson & Johnson vaccine, those signals led to a preference for the Pfizer and Moderna vaccines.

"I am confident that these vaccines are safe, including for kids. I am much more concerned about the side effects of COVID-19 than the vaccines."

The 'Mouse Test'

FDA Commissioner Dr. Robert Califf wrote on X, formerly Twitter, on August 25 that clinical trial data from other bivalent mRNA COVID-19 boosters was taken into account for the new boosters.

"Bivalent and multivalent vaccines are very common and modifying a vaccine to include different virus strains often does not require a change in other ingredients," Califf wrote. "FDA has extensive experience with reviewing strain changes in vaccines, as is done with the annual flu vaccine."

Preliminary tests conducted by Pfizer in June 2022 were done on eight mice. The actual number of mice involved in the recent trials is unclear.

Data provided by Pfizer to the CDC on Tuesday indicated trials involving female mice, 10 per group, were provided a primary series of monovalent vaccine, a third booster dose of the bivalent (original vaccine plus BA.4/5) vaccine, and then a fourth booster dose of either a bivalent (original plus BA.4/5) or a monovalent (XBB.1.5) vaccine.

A spokesperson for Pfizer referred Newsweek to the data it presented to the CDC on Tuesday, declining to comment otherwise. The CDC referred Newsweek to the FDA for comment.

Newsweek also reached out to Moderna via email for comment.

Wohl said these new boosters use the same technology and process as the previous vaccines "and only tweak the text of the message they send to our cells so that they make a COVID-19 spike protein that looks more like the variant circulating now."

"Variants have been emerging every few months as we continue to transmit the virus to each other, therefore clinical trials of each updated vaccine is not practical or helpful since by the time the results would be available newer variants would likely be circulating," he said.

Using "mouse data," as Weill Cornell Medicine immunologist John Moore told NPR, is more a concern about the efficacy on humans rather than the safety.

"For the FDA to rely on mouse data is just bizarre, in my opinion," Moore said. "Mouse data are not going to be predictive in any way of what you would see in humans."

Moore told Newsweek via email on Thursday that his remarks were given in the context of a then-ongoing discussion (in 2022) about whether mouse data predicts vaccine efficacy in humans.

"I would not have said the same words this time around, as the context is different (as is the extent of our knowledge on booster composition changes)," he said.

Wohl said such testing is par for the course, echoing Califf's reasoning that vaccines unrelated to coronavirus and SARS-CoV-2 have undergone similar clinical trials.

"We have a similar challenge with influenza," he said. "And every year the flu shot is tweaked to better match what is coming our way. Some years the match is good, others less so.

"With the mRNA technology that has been so successfully used for COVID-19 vaccines, we may be able to shorten the time to develop flu shots and therefore better match the flu virus that will arrive each flu season."

In a post-pandemic world where trust in COVID-related vaccines is likely less than what it was when they were first distributed, questions arise based not just on the safety of these new boosters but also their efficacy.

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

President Joe Biden has encouraged all eligible persons to get the booster to "protect those around you this fall and winter."

But the skeptics remain.

"The latest and greatest version of the Covid vaccine was tested on 4 rats, which oddly died peacefully of natural causes in their little cages!" wrote one user on X. "If the FDA says it's good to go, what could possibly go wrong?"

Wohl said that ultimately, everyone has a personal choice to make regarding being vaccinated. He also remains encouraged that older Americans have been the age group most willing to get vaccinated against coronavirus, which he attributes to understanding the risks associated and to those individuals' wisdom.

"These decisions should be based on facts and not lies or nonsense," Wohl said. "Unfortunately, the assault on truth that has been undertaken to cast doubt and discord, has shaken confidence in what had been respected and unassailable sources of information and advice. This has been almost as deadly as the COVID-19 virus.

"Leadership matters and when those to whom we give this responsibility point the way to what evidence tells is keeps us safe, rather than being amplifiers of our fears, we benefit. When they don't, we suffer."

Update 09/14/23, 2:17 p.m. ET: This story was updated with comment from John Moore and the FDA, as well as responses from Pfizer and the CDC.