Independent advisers to the Food and Drug Administration have recommended booster doses for both the Moderna and Johnson & Johnson COVID-19 vaccines, setting the stage for a U.S. booster program that could soon cover eligible recipients of all three vaccines now in use.

The FDA's Vaccines and Related Biological Products Advisory Committee voted Thursday to support a half-dose Moderna booster for selected adults and voted 19-0 Friday to recommend a second Johnson & Johnson dose for adults 18 and older at least two months after the first shot. The two-day FDA meeting also examined early federal data on using a different vaccine as the booster than the product used for the primary series.

Moderna's booster mirrors the Pfizer risk-based approach

Moderna is seeking authorization for a 50-microgram booster, half the dose used for each shot in its primary series. The company began its FDA submission in September, reporting that antibody levels had declined before boosting and rose substantially after a third dose.

The advisory panel's recommendation generally tracks the groups already eligible for a Pfizer-BioNTech booster: adults 65 and older, younger adults at high risk of severe COVID-19, and adults whose occupational or institutional exposure puts them at greater risk. The FDA authorized that Pfizer booster framework on September 22, describing the eligible populations in its authorization.

The Moderna discussion focused on evidence of waning antibody levels, the continuing protection of the primary series against severe disease and the question of whether a smaller booster dose provides sufficient additional immunity with fewer side effects.

Johnson & Johnson presents a different question

The Johnson & Johnson vaccine was authorized as a single-dose primary regimen, so its booster debate is not directly comparable to the two-dose mRNA vaccines. The company asked the FDA earlier this month to authorize an additional dose for adults, citing data from its ENSEMBLE 2 study. Its October 5 submission reported higher protection after a second dose given roughly two months after the first.

On Friday, the advisory committee voted unanimously to recommend authorization for a booster at least two months after initial vaccination. Johnson & Johnson said the 19-0 vote was supported by clinical data showing stronger protection after the additional dose.

The broader public-health question is whether the second Johnson & Johnson shot should be understood primarily as a booster or as a practical second dose in a regimen that produces stronger immunity with two exposures. Advisers repeatedly returned to the comparatively lower antibody levels measured after one Johnson & Johnson dose and to evidence that protection remains strongest against hospitalization and death.

Mix-and-match data could simplify the program

The FDA announced in advance that the panel would discuss heterologous, or “mix-and-match,” boosting. Its October 1 notice said advisers would review data on administering a booster made by a different manufacturer from the vaccine used initially.

That approach could give clinicians and patients more flexibility, particularly when the original vaccine is unavailable or when emerging evidence suggests a stronger immune response from another product. Federal researchers have been studying combinations of the Pfizer-BioNTech, Moderna and Johnson & Johnson vaccines, although the evidence is still limited compared with the much larger datasets supporting the original vaccine series.

The FDA's meeting materials include company and agency briefing documents detailing antibody responses, safety data and the tradeoffs regulators are considering. Those documents make clear that booster policy is being built on several kinds of evidence rather than one definitive endpoint.

Authorization is only the next step

The advisory votes are recommendations, not final federal policy. The FDA must decide whether to amend the emergency use authorizations, and the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices must then recommend how the newly authorized doses should be used.

The process is occurring while the Delta wave is receding nationally but vaccination remains uneven. Federal officials continue to emphasize that getting unvaccinated Americans through an initial vaccine series will prevent more severe illness than providing additional doses to people who already retain substantial protection.

At the same time, the booster debate has moved rapidly from whether any third doses are needed to how different products should be used for different populations. The Pfizer authorization established the first risk-based framework. This week's votes could extend it to Moderna recipients and create a broad second-dose recommendation for Johnson & Johnson recipients.

If the FDA and CDC follow the advisory committee's direction, the United States will soon have booster options tied to all three vaccines — and may also allow eligible people to receive a different product from the one they received first. That would mark a significant shift from the rigid product-specific vaccination schedules used during the first year of the campaign.