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A second booster dose of Pfizer or Moderna's COVID-19 vaccine was authorized for adults ages 50 and up, as well as certain populations at risk for severe disease, the .

In a highly anticipated emergency use authorization (EUA) that was previewed for weeks, older adults may receive a second booster dose at least 4 months after their first booster dose of vaccine, the agency said.

Other populations eligible to receive a second booster dose of the vaccines include individuals with moderate to severe immunocompromising conditions, such as solid organ transplant recipients, or those "living with conditions that are considered to have an equivalent level of immunocompromise" -- ages 12 and up for Pfizer, or 18 years and up for Moderna. This means that immunocompromised people will receive five doses of mRNA vaccine in total: a three-dose primary series, and two boosters.

At a media briefing, Peter Marks, MD, PhD, director of FDA's Center for Biologics Evaluation and Research, noted that waning immunity and risk of severe disease were the main reasons for wanting an additional dose for these at-risk groups.

"FDA believes this option will help save lives and prevent severe outcomes among our highest-risk patients," he said.

Marks said a second booster was particularly important for immunocompromised individuals because they experience "more rapid waning of immune responses and they are generally more at risk for severe outcomes of COVID."

Once again, the safety data to support this EUA was real-world data from Israel. For the Pfizer vaccine, data on 700,000 adults ages 18 and up who received a second booster at least 4 months later "revealed no new safety concerns."

While the Israeli data mostly included adults ages 60 and up, Marks said that it is "somewhat challenging to operationalize what high risk from serious COVID" means. Because one-third of adults ages 50 to 65 have "significant medical comorbidities," the agency "felt like we would capture the population with the most benefit from a fourth dose," he said.

Marks noted that the 4-month interval between boosters also came from Israeli data.

For Moderna, safety was not only "informed by experience" with the Pfizer vaccine, but a separate study of 120 adults who received a Moderna booster after receiving a two-dose Pfizer primary series and booster, the FDA said.

Immunogenicity data also came from Israel, from an open-label study of healthcare workers who either received a Pfizer or Moderna booster after receiving a two-dose Pfizer series and booster. The study showed increased neutralizing antibody levels against SARS-CoV-2, including Delta and Omicron, 2 weeks after the second booster, which was greater than at 5 months following an initial booster.

When asked why the FDA did not convene its Vaccines and Related Biological Products Advisory Committee (VRBPAC) prior to this authorization, Marks said it was a "relatively straight-forward decision" in terms of the "survival benefit" and reduction in hospitalization.

"It's always a benefit/risk discussion, and the risk associated with an additional booster is very low," he added. "If it were my relatives, I would be sending them out to do this because of the higher level of protection."

VRBPAC will be meeting on April 6 to discuss boosters for other populations who are less at risk of severe outcomes, Marks said.

"It would not be surprising if there is a potential ... need for people to get an additional booster in the fall along with a more general booster campaign," he noted, adding that VRBPAC will also be discussing variant-specific vaccines.

At press time, the CDC's Advisory Committee on Immunization Practices had yet to schedule a meeting to determine recommendations for this second booster.

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