The U.S. is one step closer to having new COVID-19 booster shots available as soon as this fall.

On Monday, the drugmakers Pfizer and BioNTech announced that they've asked the Food and Drug Administration to authorize an updated version of their COVID-19 vaccine — this one designed specifically to target the omicron subvariants that are dominant in the U.S.

More than 90% of cases are caused by the BA.4 and BA.5 subvariants, which took off this summer, but the vaccines being used were designed for the original coronavirus strain from several years ago.

Pfizer and BioNTech said they have submitted pre-clinical data on vaccine efficacy to the FDA, but did not share the data publicly.

The new "bivalent" booster — meaning it's a mix of two versions of the vaccine — will target both the original coronavirus strain and the BA.4 and BA.5 omicron subvariants.

If the vaccine is authorized by the FDA, distribution could start "immediately" to help the country prepare for potential fall and winter surges of the coronavirus, Pfizer CEO Albert Bourla said in a statement.

Following the FDA's guidance, the data the drugmakers are submitting represents a departure from what's been used in earlier vaccine authorizations.

Instead of waiting for results from human trials, the FDA asked the drug companies to initially submit only the results of tests on mice, as NPR reported last week. Regulators will rely on those results — along with the human neutralizing antibody data from earlier BA.1 bivalent booster studies — to decide whether to authorize the boosters.

"We're going to use all of these data that we've learned through not only this vaccine but decades of viral immunology to say: 'The way to be nimble is that we're going to do those animal studies," Deepta Bhattacharya, an immunobiologist at the University of Arizona College of Medicine in Tucson, told NPR recently. "We're really not going out too far on a limb here."

Pfizer and BioNTech also report that they expect to start a human study on the safety and immunogenicity of the BA4/BA5 bivalent vaccine this month.

Earlier this year, vaccine makers presented U.S. and European regulatory authorities with an option for a bivalent vaccine that targeted an earlier version of the omicron variant, BA.1. While the plan was accepted in the U.K., U.S. regulators instead asked the companies to update the vaccines to target the newer subvariants.

Scientists say the development of COVID-19 vaccines may go the way of flu vaccines, which are changed every year to try to match the strains that are likely to be circulating.

NPR's Rob Stein contributed to this report.