Pfizer BioNTech said on Monday they had sought U.S. authorization for a COVID-19 vaccine booster retooled to target the Omicron variant, and would have doses available to ship immediately after regulatory clearance.

The request to the U.S. Food and Drug Administration (FDA) was for a so-called bivalent vaccine containing the dominant BA.4/BA.5 variants of the virus along with the original coronavirus strain. It is intended for ages 12 and above.

The FDA had asked vaccine makers in June to tweak their shots to add protection against the newest Omicron variants. 

Moderna is expected to file a similar application soon for updated boosters for adults.

The U.S. has a contract to buy 105 million of the Pfizer doses and 66 million from Moderna, assuming FDA gives the green light.

BA.5 is the dominating strain causing for nearly 90 percent COVID-19 infections in the U.S., according to the weekly "Nowcast" update provided by the U.S. Centers for Disease Control and Prevention (CDC) last week.

Pfizer previously announced results from a study that found its earlier Omicron tweak significantly revved up antibodies capable of fighting the first Omicron version, BA.1, and to a lesser degree the genetically distinct BA.4 and BA.5 Omicron relatives. Its application to the FDA contains that data plus animal testing of the newest recipe update.

Pfizer and BioNTech expect to start a trial using the BA.4 and BA.5 boosters in the coming weeks, to shed additional light on how well tweaked shots work. Moderna has begun a similar study although full results wouldn't be available before a fall booster campaign.

"It's clear that none of these vaccines are going to completely prevent infection," said Dr. Rachel Presti of Washington University in St. Louis, a researcher with the Moderna trial.

Pfizer and Moderna aren't the only vaccine options in the United States. The country's health authorities just expanded the use for Novavax, a more traditional, protein-based COVID-19 shot, to people as young as 12. Novavax also plans to offer a variant update later in the year.

(With input from agencies)