Expert advisers to the Food and Drug Administration on Wednesday began sketching the outlines of a long-term strategy for deploying vaccine booster shots against the coronavirus amid uncertainty about future variants and a rapidly closing window to prepare for a potential fall vaccination campaign.
But with vaccine companies and the National Institutes of Health still conducting human testing of possible booster shots, the expert committee could not review the wisdom of any particular approach. The goal of the meeting was to provide the FDA clarity on the criteria for boosters that would be needed going forward, and Marks promised they would meet again in coming months.
Second boosters of the original Pfizer-BioNTech and Moderna vaccines were made available last week to older adults and to immunocompromised people, a stopgap move intended to shore up protection against severe disease in vulnerable people, given the unknowns about the coming months.
Many experts say that additional shots for the general population should be reformulated to keep up with the changing virus, but the question of whether, when and how to do that remains unresolved.
Results of the clinical trials for reformulated shots will offer important clues but may not provide the clarity many people are seeking on the best strategy. Even experts on viral evolution cannot predict with certainty what version of the coronavirus may be circulating in the winter.
One of the booster trials, launched last week by NIH, tests different shots. The study includes an additional dose of the original Moderna vaccine, a shot targeting the omicron variant and bivalent vaccines tuned to fight two variants at once — including beta and omicron, delta and omicron, and omicron and the original version of the virus.
The path forward is not clear-cut. An omicron-specific shot might perform better in theory, given that omicron is dominant and could spawn future variants. But several scientists pointed out that a switch in vaccine formulation could lead the immune system astray, because future variants may not be offshoots of omicron. When the delta variant was dominant, for example, many people assumed the next variant of concern would be delta-like, but omicron was a whole different beast.
Drug company executives have predicted that the next booster shot might target two versions of the virus, an approach designed to generate longer-lasting protection. But data has yet to show such shots provide better protection.
Paul A. Offit, a vaccine expert at Children’s Hospital of Philadelphia, said federal authorities should be directing the strategy on boosters.
“I feel at some level the companies sort of dictate the conversation,” Offit said. “You hear the company now has an omicron-specific vaccine. It really shouldn’t come from them, it has to come from us.”
Government officials are cognizant that a decision must be made soon about the composition of shots so companies have time to show vaccines are safe and effective and to manufacture enough shots.
For influenza, a decades-long, globally coordinated process dictates selection of a vaccine each year tuned to fight strains predicted to be circulating in fall and winter. In the Northern Hemisphere, this happens in late February, about six months before flu shots are first given.
No similar calendar has emerged for updating shots against the coronavirus, and while companies are working on potential booster shots, those efforts are not centrally coordinated.
Robert Johnson, director of the medical countermeasures program at the federal government’s Biomedical Advanced Research and Development Authority, described the timeline that would be needed for rolling out an updated shot.
Johnson said that if authorities want to have a new booster available in September, the components of that shot would need to be selected by early May so clinical trials could begin. Otherwise, he said, “it’s going to be very difficult to have, collectively across manufacturers, enough product to meet that demand.”
And while experts talked about using the influenza vaccine program as a model, they noted the perils of betting that the coronavirus will behave the same, given the many unknowns.
Hayley Gans, a pediatric infectious-disease specialist at Stanford Medicine, said experts are grappling with an unsettled environment while “trying to move forward.”
One of the many complications with the coronavirus is that it is not clear whether future waves will follow a seasonal pattern. Experts say another surge is likely in fall or winter — at the same time that immunity from vaccination and infection is waning, people are moving indoors and the virus is continuing to evolve. But the virus has yet to develop a predictable seasonal pattern, making the timing of the booster campaign a best guess.
The evolutionary path the virus will follow is also unclear.
Trevor Bedford, a scientist who studies the evolution of viruses at Fred Hutchinson Cancer Research Center, said that in 2021, SARS-CoV-2 mutated much faster than influenza viruses such as H3N2.
“In two years’ time since the start of the pandemic, we’ve accomplished about five years of equivalent evolution of H3N2,” Bedford said. “The evolution has been remarkably fast so far.”
The coronavirus has also proved especially adept at making evolutionary leaps, such as the emergence of the heavily mutated omicron variant. It remains uncertain whether and how often the virus could make such leaps in the future.
FDA official Doran Fink advised the expert committee that it was important that the case for further booster shots needed to be sensible, practical and understandable, because while a vocal segment of the public remains eager for additional shots, many Americans have yet to receive an initial booster.
“The what and the why of the recommendation should be readily apparent to patients, health-care providers, and state and local public health authorities, which is critical to achieving buy-in and avoiding confusion,” Fink said. “We all recognize how challenging it has been to consistently hit on all these objectives while synthesizing rapidly emerging and evolving data, time and time again.”