In a unanimous vote, the 21 members of the U.S. Food and Drug Administration (FDA)’s vaccine committee recommended an update to the COVID-19 vaccine to better match the viral strains currently circulating in the country and around the world.

The panel voted to move away from the current bivalent vaccine, which is designed to tackle the original virus and BA.4 and BA.5 variants, to a vaccine that is better able to protect against the XBB family of variants. The half a dozen different versions of XBB viruses account for nearly all new infections in the U.S., with two—XBB.1.5 and XBB.1.16—currently dominating.

Deciding which strain to include

The committee’s vote was based on several lines of evidence showing that the protection offered by existing vaccines wanes against newer virus variants. While the vaccines continue to protect against severe disease and death, better matching the currently circulating versions of the virus to the strains targeted by the vaccines would provide more protection against infection as well. The committee’s recommendation to update the COVID-19 vaccine is in line with a number of international public health groups’ guidance; in May, both the World Health Organization (WHO)’s COVID-19 vaccine advisory group and the International Coalition of Medicines Regulatory Authorities also recommended moving to an updated COVID-19 vaccine that targets a single XBB strain, and the European Center for Disease Prevention and Control and the European Medicines Agency in June also jointly concluded that an XBB strain is “adequate to ensure cross-reactivity against current dominant and emerging strains.”

While many public-health experts had predicted that any vaccines following the initial shots first developed and deployed at the end of 2020 might be more effective if they generated protection against more than one strain of the virus, the committee saw studies from the three vaccine makers showing that prototype shots tested in animals that targeted two XBB strains did not produce appreciably higher levels of antibodies to neutralize the major XBB viruses than shots that targeted only one XBB strain. Moderna also provided early data in about 100 people who received the company’s experimental XBB.1.5 vaccine as a booster dose, either alone or combined with company’s BA.4/5 vaccine, and the monovalent shot produced higher levels of virus-fighting antibodies against both the XBB.1.5 and XBB.1.16 viruses compared to the bivalent vaccine.

Toward a new dosing schedule

While the committee voted to update the vaccine to include an XBB variant, the experts were still divided over how often the COVID-19 vaccine should be given in the U.S. In previous meetings, the committee leaned toward an annual shot—similar to the flu shot—which would be ready before the fall respiratory disease season and would be an easy way for people to remember to get a COVID-19 vaccine. But because the virus mutates so quickly, health officials may need to monitor changes in variants more frequently. Kanta Subbarao, chair of the WHO’s strain selection committee, told the FDA’s advisory committee that her group plans on meeting twice a year to ensure that the COVID-19 vaccine doesn’t fall too far behind the ever-evolving virus.

Some members of the FDA panel agreed that it’s important to monitor, and potentially update, COVID-19 vaccines more frequently, and expressed discomfort with the idea of yoking COVID-19 vaccination to flu vaccination schedules. They noted that there is no definitive data showing that SARS-CoV-2 follows a seasonal pattern like influenza does, given how rapidly and prodigiously it produces new variants that quickly replace previous ones. “I don’t know that we know the seasonality of SARS-CoV-2 yet,” said Pamela McInnes, former deputy director of the National Center for Advancing Translational Sciences at the National Institutes of Health. “Maybe we are going to be [continually] chasing this one.”

Other experts, including scientists from the U.S. Centers for Disease Control and Prevention (CDC) and the WHO, pointed to the fact that the virus seems to be evolving and mutating in a linear, potentially more predictable fashion, and not picking up old mutations or reverting to earlier strains. That could make the vaccine easier to update on a more regular basis.

Dr. Peter Marks, director of the Center for Biologics Evaluation & Research at the FDA, countered with a practical argument, noting that even if variants change more frequently than every 12 months, vaccine makers and health officials would need time to both test candidates in animals and in the lab for safety and efficacy, and to manufacture sufficient numbers of doses. He also focused on the urgent need to provide the public a more regular framework with which to approach COVID-19 vaccination, rather than the haphazard nature that’s driven the release of new COVID-19 vaccines over the last three years. More consistency and reliability for the public would hopefully lead to higher numbers of people getting vaccinated. “We have not done a good job of communicating to the American public what is going on because they are still not getting these vaccines in the ways we would like to see,” he said.

Determining who needs new shots

Some members brought up another issue: whether everyone should be getting the updated vaccine. People now have substantially higher levels of immunity than they did when the vaccines were first deployed—both from vaccinations, boosters, and COVID-19 infections. That protection comes not only from antibodies against specific variants, but also from T cells that provide longer-lasting and broader protection against a larger number of virus variants. T cell-based immunity is the reason why people with prior exposure to the virus are still largely protected from severe disease, hospitalization, and death, even if they may not still have appreciable levels of antibodies against the latest variants.

Because T-cell immunity is conserved, Dr. Paul Offit, professor of pediatrics at Children’s Hospital of Philadelphia, questioned whether everyone would need to get an updated vaccine on a regular basis. “We need to continue to define who those at highest risk of severe disease are,”—including the elderly and those who are immunocompromised—”and who may benefit from a booster, and not make this a recommendation for everybody for every season.” Not everyone will necessarily benefit from a booster, he said.

The decision about who should get vaccinated, and how often, will fall to the CDC’s immunization committee, which will meet once the FDA makes a final decision based on the panel’s recommendation to update the COVID-19 vaccine.

If the FDA does greenlight an updated vaccine, manufacturers say they’re ready to produce enough doses to start immunizing people in the fall. Both Moderna and Pfizer-BioNTech rely on new mRNA technology which has the advantage of speed over older technologies, and could lead to new vaccines in as soon as six weeks. Most of the manufacturers have begun making versions of the XBB.1.5 vaccines, and Pfizer-BioNTech told the committee it could deliver XBB.1.5 vaccines as early as July, an XBB.1.16 vaccine by August, and any other formulation by October. While Moderna did not reveal its timeline, because it uses the same technology, the company would likely be able to manufacture doses in a similar timeframe. Novovax, however, uses a different platform that involves creating recombinant viral proteins, so the company said that it would take about six months to produce sufficient doses of an XBB.1.5 vaccine.