By Ingrid Hein

The FDA on Wednesday authorized bivalent COVID-19 booster shots from Pfizer/BioNTech and Moderna that target the original strain along with the Omicron BA.4/BA.5 subvariants.

In an amendment to the emergency use authorizations (EUAs), both shots can be administered starting 2 months following a primary vaccination series or prior booster -- in people 12 and up for Pfizer's shot and in people 18 and up for Moderna's vaccine.

"The FDA has been planning for the possibility that the composition of the COVID-19 vaccines would need to be modified to address circulating variants," Peter Marks, MD, PhD, of FDA's Center for Biologics Evaluation and Research, said in a statement. "We sought input from our outside experts on the inclusion of an omicron component in COVID-19 boosters to provide better protection against COVID-19. We have worked closely with the vaccine manufacturers to ensure the development of these updated boosters was done safely and efficiently."

Of note, clinical data supporting the EUA amendment were derived from studies on bivalent vaccines that targeted the original Omicron strain, but the agency said it considers such data as "relevant and supportive" of the newer BA.4/BA.5-targeted vaccines.

Support for the vaccines' efficacy came in the form of immunogenicity data. The studies for each vaccine involved about 600 fully vaccinated adults who had already received a booster shot as well. Immune responses to BA.1 were superior in individuals receiving either of the companies' bivalent vaccines -- which included components targeting the ancestral strain as well as Omicron BA.1 -- compared to those that received the original monovalent boosters targeting only the original strain.

This rollout is "similar to situations we've done in the past with influenza strain changes ... we don't do clinical studies for them in the United States," Marks said during a press briefing on Wednesday.

"To stave off serious outcomes and symptomatic disease, one needs to refresh the immune system with what is actually circulating," he said. "BA.1 is no longer circulating."

CDC's variant proportions tracker shows that the BA.5 Omicron subvariant makes up 88.7% of cases in the U.S., while BA.4 makes up 3.6% and BA.4.6 makes up 7.5% of cases.

Prior research in individuals infected with BA.4 or BA.5 has demonstrated robust protection against a variety of other variants, including BA.1, Delta, and Beta, Marks noted.

"The hope here is that by increasing the amount of antibodies we have to that particular variant, we will restore the protection we had when these [vaccines] were launched -- that is the goal here," he said.

Safety data for Wednesday's authorization were derived from studies on the BA.1-targeting bivalent vaccines in 800 adults 18 and up for Moderna's shot and in 600 adults 55 and older for Pfizer's. Commonly reported side effects for the vaccines included injection site pain, redness, and swelling, lymph node swelling in the arm of the injection, as well as chills, fever, fatigue, headache, muscle or joint pain, and nausea or vomiting.

"We are confident in the evidence supporting these authorizations," he said.

Monovalent mRNA vaccines that target the original SARS-CoV-2 strain will no longer be authorized as boosters in people 12 and up, the agency said.

Marks said he expects the FDA will authorize bivalent boosters for children in the next month or two.

Before shots can go into arms, the CDC's Advisory Committee on Immunization Practices (ACIP) will need to weigh in with their recommendation; ACIP is scheduled to meet on Thursday and Friday of this week.