The Vaccines and Related Biological Products Advisory Committee voted unanimously to replace the original monovalent formula, protective against the initial Wuhan strain, with the bivalent formula, which protects against both the Wuhan strain and Omicron BA.4 and BA.5, for those receiving their initial shots.

When bivalent boosters became available in the U.S. around Labor Day, Americans became ineligible to receive monovalent boosters. But the monovalent formula has continued to be used in so-called “primary series” shots—two in the case of Moderna, and up to three in the case of Pfizer.

“Simple is better, and frankly I think we saw today that closer is better,” committee member Dr. Michael Nelson, chief of the Asthma, Allergy, and Immunology Division at the University of Virginia School of Medicine, said after the vote. “It relieved my fears that we’d be in this game of trying to chase the latest [variant]. Today we saw reassuring data that shows just getting [the vaccine] closer [to currently circulating variants] gives additional benefit.”

The advisory committee’s recommendation must be approved by the U.S. Centers for Disease Control and Prevention before implementation.

A controversial proposal

The aim of Thursday’s meeting was to “determine if we’ve reached a point in the pandemic where we can simplify and take a more routine approach” to COVID vaccine development and dosing, said Dr. David Kaslow, director of the Office of Vaccines Research and Review (OVRR) in the Center for Biologics Evaluation and Research (CBER) at the FDA.

Such simplification starts with the use of the same formula for both initial and booster vaccines. But the committee could eventually recommend further steps, like the administration of one booster shot a year for most Americans, updated annually for distribution in the fall. Two or more annual doses may be recommended for those who are at higher risk of severe disease and death, like the elderly and immunocompromised.

The committee could meet at any time to discuss an additional booster for a “highly pathogenic escape variant,” should one occur, Kaslow added.

The federal agency’s proposal is a controversial one—not for lack of a desire for a predictable COVID vaccine schedule, akin to the one we have for flu, but due to concerns that the shift may be premature.

“Unlike the flu, global distribution of [COVID] variants is more variable,” Jerry Weir—director of the Division of Viral Products at the FDA’s OVVR—said at the meeting, referencing the continual, highly unpredictable rise and fall of various COVID variants in different regions of the globe.

“Going forward, it’s still very challenging. [COVID] variants don’t sweep across the world like they seem to with influenza,” he added. Strains of flu dominant in the Southern Hemisphere’s winter usually become dominant during the Northern Hemisphere’s winter, allowing for relatively easy annual vaccination planning. Not so with COVID variants, some of which cause massive waves in some countries but barely make a splash in others, for reasons science hasn’t quite figured out.

“But our primary responsibility is what’s best for the U.S. market, and that’s where our focus will be.”

Dr. Peter Marks, director of the FDA’s CBER, said that COVID seems to be developing some seasonality, with nearly guaranteed spikes in the winter due to increased indoor gatherings. If Americans can be convinced to get their flu shot along with an annual COVID booster, such a transition “seems like a reasonable way to go,” he said. Pfizer, Moderna, and Novavax all have studies underway of flu and COVID one-shot combos, representatives for each company said at Thursday’s meeting.

Others committee members, however, pointed out that annual summer surges also seem to occur, which could lead to the need for boosters more than once a year. And others yet pointed said annual boosters may not even be necessary. “It seems to be awful early in this process to answer that question,” Dr. Cody Meissner, professor of pediatrics and medicine at the Geisel School of Medicine at Dartmouth-Hitchcock Medical Center, said.

Not quite the flu yet—if ever

Several committee members expressed concerns that the FDA is attempting to turn COVID—a virus distinct from the flu—into the flu, forcing it to fit into an annual vaccination model that may not be appropriate now—or ever.

A committee meets meets each February to choose strains of flu to include in the annual Northern Hemisphere vaccine. “If we miss with the vaccine strain and don’t get the circulating strain, you have pretty much no protection” against flu, Dr. Paul Offit, professor of pediatrics in the Division of Infectious Diseases at the Children’s Hospital of Philadelphia, said.

“This isn’t true with this virus,” he added, referencing studies that have shown that Omicron boosters released last fall are still providing good, although reduced, protection against severe disease and death from new variants like BQ.1.1 and XBB.1.5.

While 81% of Americans have received at least one dose of COVID vaccine, only 15% have received an updated Omicron booster. It’s a statistic that worries public health officials, as antibody immunity from vaccination and infection is thought to wane after about three months. T-cell immunity appears to last much longer and is thought to be responsible for the broad level of population immunity seen in the U.S. and other countries with high rates of vaccination and infection.

A whopping 95% of Americans 50 and older are thought to be vaccinated, Dr. Heather Scobie, a CDC epidemiologist, told attendees at Thursday’s meeting. In contrast, the vast majority of U.S. children 4 and younger—90%—are unvaccinated.

The FDA advisory committee will meet again around May to discuss the possible composition of a fall COVID booster, Dr. Peter Marks, director of the FDA’s CBER, said.

The committee will meet roughly once a year, barring a new COVID strain that breaks through the barrier of T-cell immunity, he added.

“We totally agree with everyone—this isn’t the flu,” he said. “We can take the best of that model and adjust around it.”