Pfizer asked U.S. regulators Monday to authorize its combination COVID-19 vaccine that adds protection against the newest omicron relatives — a key step toward opening a fall booster campaign.

The Food and Drug Administration ordered vaccine makers to tweak their shots to target BA.4 and BA.5 that are better than ever at dodging immunity from earlier vaccination or infection.

If the FDA quickly clears the combo shots made by Pfizer and its partner BioNTech, boosters could be offered within weeks. The U.S. has a contract to buy 105 million of the updated Pfizer doses as soon as health authorities greenlight them, and the company said doses are ready to ship.

Moderna is expected to file a similar application soon, and the U.S. has a contract to buy 66 million doses of its updated vaccine.

“It’s going to be really important that people this fall and winter get the new shots. It’s designed for the virus that’s out there,” White House COVID-19 coordinator Dr. Ashish Jha said last week.

For now at least. BA.5 currently is causing nearly all COVID-19 infections in the U.S., and much of the world. There’s no way to know if it still will be a threat this winter — or if another mutant will have replaced it.

The news comes after Britain a week ago became the first in the world to authorize a different update to Moderna’s COVID-19 vaccines — shots that add protection against the original omicron that struck last winter.

The U.S. opted not to use that earlier tweak to the vaccine — setting up a fall where different countries will be using different versions of booster shots to rev up protection against another possible winter surge.