“As we head into fall and begin to spend more time indoors, we strongly encourage anyone who is eligible to consider receiving a booster dose with a bivalent COVID-19 vaccine to provide better protection against currently circulating variants,” Califf said in a statement.

The updated emergency use authorizations permit administration of a single booster at least two months after a primary or booster dose is given. Moderna’s is authorized for all U.S. adults age 18 and up, while Pfizer’s is permitted for anyone 12 and older.

The FDA landed on a two-month minimum window for space between a previous dose and a dose of the updated booster to optimize an individual’s immune response and to hedge against the risk of a side effect, such as myocarditis, Marks said, which has been linked to messenger RNA vaccines, particularly in males ages 12 to 39 when shots are given in a short timeframe.

“In terms of trying to stave off serious outcomes and symptomatic disease, one needs to refresh the immune system with what is actually circulating. It’s a benefit-risk [balance] here,” Marks told reporters.” We believe the benefits of receiving a boost now at least two months after are going to outweigh the risks.”

Marks noted that many Americans who would be newly eligible for the updated boosters would also be several months removed from their last vaccine dose.

Marks and Califf also pointed to emerging data suggesting immunization can reduce an individual’s risk of developing long Covid, a debilitating post-disease condition marked by myriad symptoms that include fatigue and brain fog.

“It’s convincing that the more up-to-date you are on your vaccination — and likely the more updated your vaccination for this variant — the greater the chance that you’ll be protected with a reduced risk of long Covid, which for young people is increasingly a major concern,” Califf said.

The updated emergency use authorization means the original formulations of the Moderna and Pfizer-BioNTech vaccines are no longer authorized as booster doses for anyone 12 and older, the FDA said.

Pfizer said in a statement Wednesday that it plans to withdraw its supplemental biologics license application for its original vaccine as a booster dose for people 16 and older and will refile for the bivalent option. The original vaccine will remain available as a booster dose for kids ages 5-11.

Both booster shots contain messenger RNA that codes for the spike protein of the original SARS-CoV-2 variant, as well as the spike protein of the Omicron variant’s BA.4 and BA.5 subvariants, which are circulating now. The boosters will be available only for people who have received at least a full primary vaccination series, according to a federal planning guide. The same planning guidance said it expects bivalent boosters for children 11 and younger to be authorized “within a short time.”

Data dilemma: The FDA based the boosters’ authorization on real-world data for existing mRNA vaccines, human trial data from other bivalent shots authorized in other parts of the world and data from rodent studies. Some public health experts have argued that it is too soon to green-light vaccines without safety and efficacy data for people, though the annual flu vaccine is authorized using data from animal testing because of the quick turnaround time needed every year. The flu vaccine is not based on the newer mRNA technology however.

What’s next: The Biden administration has ordered 175 million doses of the new boosters. Pfizer has said it can ship up to 15 million doses of its booster by Sept. 9.
Pfizer said Wednesday it plans to apply for an emergency use authorization for its bivalent booster to be used in kids ages 5-11 in early October, and that it’s working with the FDA to prepare an application for the shot to also cover babies and toddlers.