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Certain groups are now eligible for another dose of the bivalent COVID-19 vaccine, and the updated shots have now fully replaced the monovalent shots, the on Tuesday.

The agency scrapped the emergency use authorizations (EUAs) for the monovalent Moderna and Pfizer-BioNTech mRNA vaccines, and has authorized the bivalent boosters (original plus Omicron BA.4/BA.5 strains) for all doses starting at age 6 months.

People age 65 and up can now get a second bivalent dose at least 4 months after their initial bivalent dose. FDA said the second dose is supported by data showing that immunity wanes in this population over time, but that an additional dose restores it.

Those who are immunocompromised can get another bivalent dose at least 2 months after their initial bivalent shot, FDA said. They can have additional doses on top of that at the discretion of their healthcare provider, who can also determine the interval at which these doses are received, the agency said.

For young kids (ages 6 months through 4 years) who are immune compromised, however, eligibility for additional doses will depend on the vaccine they previously received, the agency noted.

At a press briefing announcing the changes, Peter Marks, MD, PhD, director of FDA's Center for Biologics Evaluation and Research, said more detailed recommendations for additional booster doses for immunocompromised individuals will be discussed at the CDC's Advisory Committee on Immunization Practices (ACIP) .

Outside of those groups, most people who only had monovalent shots in the past can now get a single dose of a bivalent booster dose. The agency intends to make decisions about future vaccination after a meeting of FDA's Vaccines and Related Biological Products Advisory Committee (VRBPAC) in June regarding the fall strain.

Marks said the current bivalent vaccine appears to help prevent severe disease from XBB.1.5 infections, currently the in the U.S., accounting for 78% of all cases.

The agency is also closely watching XBB.1.9.1 (so-called Hyperion) and XBB.1.16 (Arcturus), both of which account for 7% of cases each, according to CDC's Nowcast variant proportions tracker.

"The same spike protein is present on XBB.1.5 and in those other variants, to a large extent," Marks noted. "It may turn out that as we go to select [the next vaccine], we will be able to find something that is able to cover them all -- but that will be the topic of the discussion in June."

As for unvaccinated adults, the new EUAs state that a single dose of bivalent vaccine will suffice as initial vaccination, rather than two doses of the monovalent vaccines, according to the agency. The same goes for unvaccinated kids 5 years and up (with Pfizer's vaccine) or 6 years and up (for Moderna's).

Unvaccinated kids ages 6 months through 5 years can get a two-dose series of the Moderna bivalent vaccine. Unvaccinated kids ages 6 months through 4 years can get a three-dose series of the Pfizer-BioNTech bivalent vaccine.

Kids 6 months through 5 years who've already had one, two, or three doses of a monovalent vaccine can get a bivalent shot, but the number of doses will depend on the vaccine and their vaccination history, the FDA said.

The changes announced on Tuesday were based on January discussion from VRBPAC, which recommended the agency harmonize the strain composition of COVID-19 vaccines.

"Most of the U.S. population 5 years of age and older have antibodies to SARS-CoV-2, the virus that causes COVID-19, as a result of either vaccinations or having been infected," said Marks. "Data support simplifying the use of the authorized bivalent mRNA COVID-19 vaccines, and the agency believes that this approach will help us achieve higher vaccination coverage across the country."

Support for the effectiveness of a single dose of the bivalent Moderna vaccine comes from FDA's analysis of immune response data from two studies -- one in 145 people age 6 years and older who had evidence of a prior COVID infection and another in 1,376 people age 6 years and up without a prior infection, but who had two doses of the monovalent shot. Data showed that the immune response after one dose among those with prior infection was comparable to that seen after two doses among those without prior infection, the FDA said.

Effectiveness of a single dose of the bivalent Pfizer-BioNTech vaccine was in part supported by observational data from England on the effectiveness of one dose of the company's monovalent shot in adolescents, which showed additional protection for those with prior infection versus those without a past infection.

Staff writer Ingrid Hein contributed to this story.

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