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The FDA authorized booster doses of Moderna and Johnson & Johnson's COVID-19 vaccines and also authorized a "mix and match" or heterologous approach to boosters for all three available vaccines (including Pfizer's) in the indicated populations, .

Ultimately, the FDA went with what was recommended by their advisory panel, the Vaccines and Related Biological Products Advisory Committee (VRBPAC), which voted for a half-dose Moderna booster in adults ages 65 and up, adults ages 18-64 at high risk of severe COVID, and adults 18-64 with frequent occupational and institutional exposure to SARS-CoV-2, at least 6 months after completing the primary series.

A booster of Johnson & Johnson vaccine is authorized for all adults ages 18 and up who received the Johnson & Johnson vaccine, at least 2 months after completion of the single-dose regimen.

For the heterologous boosters, the agency specified that they are authorized for "eligible individuals." These individuals may receive a booster dose of a different vaccine than their primary series "with a different available COVID-19 vaccine" if they are among the populations indicated for boosters, given at the same recommended interval (after 2 months if Johnson & Johnson was the primary vaccine, or after 6 months for Moderna or Pfizer).

The agency also clarified the Pfizer booster emergency use authorization (EUA), for the 18-64 population at high occupational or institutional risk, to more closely align with the language in the Moderna booster EUA.

At a media briefing Wednesday evening, Acting FDA Commissioner Janet Woodcock, MD, said the agency was not making "preferential recommendations" on boosters, and said that "we would expect many people will get the same series they have already received."

FDA's Director for the Center for Biologics Evaluation and Research, Peter Marks, MD, PhD, added that he expected individuals would consult with their healthcare providers if they had a question about which booster to receive.

"In some situations, there may be an issue with availability or medical considerations that a provider may decide" to recommend a certain booster "for a certain individual," he said.

Woodcock added that some individuals may want another vaccine as a booster due to an adverse reaction to the primary series.

"We wanted to provide a lot of flexibility, because some people don’t know which vaccine they got," she said.

Some members of VRBPAC had expressed interest in making boosters available to all adults as young as age 40, which was echoed by several reporters, who wondered why the agency didn't issue a broader recommendation for boosters.

Woodcock said they were monitoring breakthrough infections and waning immunity in different populations. She described it as a "dynamic situation."

Marks added that the FDA is "relatively nimble" with their EUA authority, and they could act as soon as they needed to, should the situation change.

"There is evidence to suggest that lowering the age of those eligible for boosters may make sense in the future," he said.

Data to support the Moderna booster EUA came from a phase II trial, P201. Overall, 149 adults who received the two-dose 100 μg series (the dose authorized under EUA) plus the 50 μg booster dose were evaluated for immunogenicity.

The data for Johnson & Johnson's booster dose was even thinner, with four trials submitted and only part of one independently reviewed by the FDA. However, the agency statement noted that the reason for the EUA was "based on the FDA's evaluation of immune response data in 39 participants from a clinical trial including 24 participants who were 18 through 55 years of age and 15 participants who were 65 years of age and older" who received a booster dose about 2 months after their first dose and "demonstrated a booster response."

For the "mix and match" EUA, the agency cited data from a pre-print, presented by the National Institute of Allergy and Infectious Diseases, that showed heterologous boosters produced an immune response, with no new safety concerns.

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