Update (September 2): The CDC greenlighted the use of the Moderna and Pfizer Omicron boosters yesterday, meaning that the shots could start being administered within days. People must have had their most recent COVID-19 jab at least two months previously—a cutoff that several members of a panel of expert advisers to the CDC suggested was too short, according to The New York Times.

Update (September 1): The European Medicines Agency today has recommended the authorization of both the Pfizer and Moderna Omicron booster shots, which it says in a statement can both be used in people aged 12 years old and up. Meanwhile, Health Canada has approved the Moderna booster shot in people aged 18 and over.

Two sorts of booster shots designed to protect people against the latest Omicron subvariants of SARS-CoV-2 received emergency use authorization from the US Food and Drug Administration today (August 31), the agency has announced

The formulations, which target both the original SARS-CoV-2 strain and the highly infectious BA.4 and BA.5 lineages of the Omicron variant, can only be administered to people who have completed their primary two-dose vaccination. One, developed by Pfizer/ BioNTech, has been authorized for people aged 12 years old and up, while the other, developed by Moderna, has been OK’d for people aged 18 or older. 

The Food and Drug Administration (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, director of the agency’s Center for Biologics Evaluation and Research, says in the announcement. He adds: “We have worked closely with the vaccine manufacturers to ensure the development of these updated boosters was done safely and efficiently.”

The creation of Omicron-targeting vaccines emerged as a priority for the FDA earlier this year as data and expert opinion pointed to the benefits of this approach as opposed to a fourth dose of the original COVID-19 vaccines. Speaking about the choice for healthy people between getting an Omicron booster or a fourth dose of the original vaccine, University of South Carolina School of Medicine Columbia’s Prakash Nagarkatti told The Scientist earlier this month: “Clearly the data show that the bivalent vaccines which incorporate the Omicron variant provide stronger and better protection against the Omicron variants. . . . Thus, it is clearly better to get vaccinated with such newer vaccines than the original vaccines because Omicron is the primary strain that is spreading.”

The new bivalent shots contain mRNA coding for the spike protein of the original SARS-CoV-2 variant and that of the latest Omicron lineages, although the companies differ in exactly how much of each they use. On its website, Moderna describes its shot as a “50 µg booster dose . . . 25 µg of mRNA encoding for the spike protein of BA.4/.5 and 25 µg encoding for the original strain of the SARS-CoV-2 virus.” Pfizer’s formulation instead “combines 15-µg of mRNA encoding the wild-type spike protein . . . and 15-µg of mRNA encoding the spike protein of the OmicronBA.4/BA.5 subvariants.”

See “Moderna vs. Pfizer: Is There a “Best” mRNA Vaccine?

The FDA made its decision without requiring that the companies submit additional human data on the modified vaccines, employing a procedure the agency already uses to authorize other sorts of vaccines, such as flu shots, that undergo minor modifications from year to year. 

The FDA instead reviewed data that Pfizer and Moderna had collected from clinical studies of the safety and efficacy of earlier iterations of the shot, which had been targeted at the original Omicron subvariants circulating last winter, the Associated Press reports. In addition, the agency examined the results of animal tests with the new, BA.4- and BA.5-targeting boosters, which showed the formulations triggered “a very good immune response,” Marks tells the AP.

Full approval for the boosters will require human data—something that the companies plan to collect in clinical trials starting next month, Science reports. The outlet also reports that some researchers express concern that the lack of human data at this point could undermine public acceptance of the vaccines. 

The Centers for Disease Prevention and Control (CDC) will now issue its own recommendation about who should receive the boosters. The agency has scheduled a vote among its Advisory Committee on Immunization Practices for this Thursday, according to NBC.