FDA is expected to authorize Pfizer-BioNTech's and Moderna's updated Covid-19 boosters this week—and while some experts have said the "new boosters are expected to help provide greater protection against the currently circulating strains," others have warned that they are "still not exactly sure" how protective the updated boosters will be.

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Updated booster shots will soon be available

The updated booster shots from Pfizer-BioNTech and Moderna are bivalent vaccines that target the BA.4 and BA.5 omicron subvariants, as well the as the original coronavirus strain, in one shot.

Last week, both companies finalized their applications for emergency authorization of their updated boosters. Pfizer-BioNTech is seeking authorization for individuals ages 12 and older, while Moderna is seeking authorization for all adults.

According to a senior Biden administration official, the updated vaccines will go through the regulatory approval process this week and are expected to become available after Labor Day.

"When available, new boosters are expected to help provide greater protection against the currently circulating strains. We encourage all who are eligible to consider a booster," FDA commissioner Robert Califf tweeted Thursday.

However, unlike previous Covid-19 vaccines, the updated boosters have not yet been widely tested on humans. Instead, the companies submitted data on mice, as well as human neutralizing antibody data from previous studies looking at bivalent BA.1 boosters.

Some vaccine experts have argued that mice data cannot be reliably generalized to humans. But others say the country has had enough experience with Covid-19 vaccines to understand their safety and can treat them like flu vaccines, which are changed every year to match circulating strains but not routinely tested.

"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. FDA has extensive experience with reviewing strain changes in vaccines, as is done with the annual flu vaccine," Califf tweeted.

"The FDA should be commended for wanting to take an aggressive, expedient approach," said Eric Topol, EVP of Scripps Research. "But we also need to acknowledge the uncertainties about efficacy and the response from the public."

Do you need an updated booster?

The updated boosters should provide a level of protection that was not possible with the original formulation of the vaccine. However, some experts suggest that how well the shots work may be dependent on an individual's past exposure to the virus—whether through vaccination, infection, or both.

"A bivalent vaccine will have some benefit for almost everybody who gets it," said Rishi Goel, an immunologist at the University of Pennsylvania. "How much benefit that is, we're still not exactly sure." 

While the amount of protection will likely vary, many experts agree that healthy individuals should wait between three and six months from their last infection or vaccination to increase the benefits of an updated booster.

After the updated boosters are available, Peter Marks, the top FDA vaccine official, said the agency may advise that people who recently received a dose of the vaccine wait "a few months" before getting the updated shot. In addition, CDC may recommend whether recently infected individuals need to wait.

According to Marion Pepper, an immunologist at the University of Washington, individuals who have already received three or more doses "have probably maxed out the protective capacity" of the original vaccine. However, the bivalent vaccines should provide increased protection to those who have not yet been infected by the omicron variant.

Experts suggest that people who have not experienced a coronavirus infection since November 2021—and have not recently boosted—should get the new shot as soon as they are available.

Those who have been infected with omicron at some point "might still have something to gain from seeing the BA.4 and BA.5 spike proteins—especially if your goal is to avoid getting sick with COVID at all," according to The Atlantic.

However, individuals who have been boosted in the last few months may not benefit much from the added protection of an updated booster, largely because their antibody levels are still probably relatively high.

According to Goel, the boosters' impact on a person's long-term protection against severe disease is not clear after a certain number of doses. Still, "the bivalent vaccines, or really any variant-containing vaccines, have real value" for those trying to prevent infection, Goel added.

For individuals who were infected with omicron this summer, "[y]ou're still riding the wave of antibodies that you generated as a result of that infection," so the updated booster probably won't provide much added protection, said Jenna Guthmiller, an immunologist at the University of Colorado.

According to Goel, high-risk individuals should wait at least a month after their most recent infection or shot, but they do not need to wait for three months if they are concerned about their risk, and should ultimately get their booster as soon as it's available to them. "I'm honestly in favor of recommending boosting as a way to maximize individual benefit," he said.

Ultimately, Guthmiller and Goel said they were not concerned about the lack of human data for the updated shots, adding that the bivalent shot is almost certainly better than the original. (Owens, Axios, 8/28; Gutman-Wei, The Atlantic, 8/27)