What will the next round of Covid booster shots look like?
The Food and Drug Administration is hoping to have an answer Tuesday, when its advisory committee will convene to review and vote on potential changes to the vaccines for the fall.
The Vaccines and Related Biological Products Advisory Committee has an incredibly short time window to make a decision: The FDA said in April that scientists needed to determine what sorts of updates the vaccines would require by the end of June, so drugmakers have enough time to produce the shots by the fall.
The panel will specifically be asked to vote on whether the agency should authorize updated booster shots that include an omicron-specific target, according to briefing documents posted online Saturday.
The existing boosters from both Pfizer-BioNTech and Moderna are still formulated to target the original coronavirus strain identified in Wuhan, China, in late 2019.
While those shots still provide a high level of protection against severe illness, hospitalization and death from Covid, the omicron variant has dealt a blow to their protection against infection.
FDA scientists said in the briefing documents that clinical trial data from Pfizer and Moderna suggest that including an omicron component in the shots appears to up the immune response to the variant.
Both companies released clinical trial data this month showing their bivalent vaccines — designed to target both omicron and the original coronavirus strain in a single shot — elicited a high level of antibodies against the original omicron strain, as well as the omicron subvariants BA.4 and BA.5.
Experts have been worried about BA.4 and BA.5, in particular, because the subvariants appear better skilled at dodging immunity and causing reinfection than previous strains.
The two subvariants have been gaining ground in the United States, making up about 35% of all new Covid cases as of June 18, according to data from the Centers for Disease Control and Prevention.
Including an omicron component in the updated boosters "makes sense," said Dr. Anna Durbin, a vaccine researcher at Johns Hopkins University in Baltimore. That's because, she said, a more specific target is needed to generate more antibodies that can attack the omicron variant specifically.
Still, in private conversations ahead of Tuesday's meeting, some committee members told top FDA officials that they were not convinced that clinical trial data showed that updating boosters with an omicron-specific target offered more protection than the existing shots, according to people familiar with the conversations.
The studies have shown the updated shots produce an improved immune response, but not whether they provide more durable, longer-lasting protection, the people said. Currently, antibodies generated from the existing booster start waning after about four months.
In the past few days, FDA officials and some panel members struggled with how to proceed with the meeting, with the latter pushing for a vote, fearing agency officials had already made a decision on whether to update the shots, the people said.
John Moore, a professor of microbiology and immunology at Weill Cornell Medical College, has remained skeptical that the bivalent vaccine will provide stronger protection than the existing booster shots.
He pointed to an animal study, posted online to a preprint server earlier this year, that suggested an omicron-specific booster may not provide any greater protection compared to the existing booster shots.
Dr. William Schaffner, an infectious diseases expert at the Vanderbilt University Medical Center, was more hopeful, saying the updated shots should provide a good level of protection against both omicron and its subvariants.
He said the strong protection could provide "assurance" to Americans, many of whom have yet to receive their first or second booster dose.
If the FDA does grant authorization to the bivalent vaccines, supply will be an issue.
Dr. Ashish Jha, the White House Covid response coordinator, said during a news briefing Thursday that due to lack of funding from Congress, there will not be enough doses for every American who wants one.