The Food and Drug Administration on Tuesday authorized a second booster dose of either Pfizer's or Moderna's COVID-19 vaccines for older adults and people whose immune systems are compromised in certain ways.
In making the decision, the FDA cited accumulating evidence that indicates "some waning of protection over time" against the disease's worst outcomes in individuals who are at higher risk. The agency cleared an initial booster shot of Pfizer's vaccine for older and immunocompromised adults last September and steadily expanded eligibility criteria over the fall to include all adults as well as recipients of Moderna's and Johnson & Johnson's vaccines.
"Based on an analysis of emerging data, a second booster dose of either the Pfizer-BioNTech or Moderna COVID-19 vaccine could help increase protection levels for these higher-risk individuals," said Peter Marks, head of the FDA office that reviews vaccines, in a statement.
The agency made its decision on authorization ahead of an advisory meeting currently scheduled for April 6. The expert panel is slated to discuss COVID-19 booster doses as well as the "process for COVID-19 vaccine strain selection to address current and emerging variants."
Under the new authorization, adults aged 50 or older who received their first booster shot at least four months prior can get a second booster dose. The FDA also cleared another booster of Pfizer's vaccine for people who are at least 12 years of age and have received an organ transplant or are similarly immunocompromised. For Moderna's vaccine, the clearance pertaining to immunocompromised individuals only applies to those 18 years or older.
The criteria for older adults are broader than what Pfizer had requested earlier this month, and narrower than what Moderna had sought.
In a Tuesday call with reporters, Marks said the FDA chose 50 years as a cut off for the authorization because one-third of people between 50 and 65 years old have health conditions that put them at higher risk, and because notable side effects like heart inflammation are less common in that age range.
The data were persuasive enough that agency reviewers didn't see the need to convene an advisory committee on these specific authorizations, he said. "This was a relatively straightforward decision."
Soon after the FDA announced its authorizations, the Centers for Disease Control and Prevention updated its recommendations to reflect the new clearances.
Both Pfizer and Moderna cited emerging data from Israel and elsewhere that suggested diminishing protection from vaccination, particularly as the omicron variant became the dominant circulating strain. The FDA also cited evidence from Israel in reaching its decision, including safety surveillance from 700,000 adults, mostly older than 60 years, who had received a fourth dose of Pfizer's vaccine.
No new safety concerns were identified in that analysis, the FDA said. For assessing Moderna's vaccine, the agency noted data from a small study of 120 participants older than 18 who received a second booster dose.
The FDA is relying heavily on antibody data, rather than evidence on clinical outcomes, to assess the added benefit of an additional shot. On that score, a study of healthcare workers at a single site in Israel found a second booster dose could increase neutralizing antibody levels against the coronavirus, according to the FDA.
Experts are still not certain exactly what level of antibodies is needed to confer protection against COVID-19 or how much of a role other immune defenders play in maintaining immunity over time. However, higher antibody levels are associated with greater protection from disease, hospitalization and death.
In the U.S., COVID-19 cases and deaths have decreased significantly in recent weeks, leading to the relaxation of public health measures across the country. But there are concerns that cases could rise again, as has happened in some European nations alongside the spread of an infectious omicron subvariant. That strain, known as BA.2, has become the most prevalent in the U.S. in recent weeks.
The FDA said it would continue to evaluate study data on vaccine protection to assess whether a second booster dose is necessary for other age groups.
Pfizer and Moderna are both testing new formulations of their vaccine tailored to the omicron variant.
Note: This story has been updated to include commentary from an FDA official and mention of the updated CDC guidance.