The dose can increase antibody levels to a point where it might only be needed annually, says Moderna’s chief medical officer.

The U.K.’s Medicines and Healthcare Products Regulatory Agency (MHRA) authorized the booster on Monday.

“We are delighted with the MHRA’s authorization of Spikevax Bivalent Original/Omicron, our next-generation COVID-19 vaccine,” said Stéphane Bancel, CEO of Moderna in a statement. “This bivalent vaccine has an important role to play in protecting people in the U.K. from COVID-19 as we enter the winter months.”

The company says it expects further authorization decisions from other countries in the coming weeks.

Whether that will occur in the U.S., however, is a complicated matter. The Moderna booster that received approval in the U.K., known as mRNA-1273.214, targets the BA.1 Omicron mutation, not BA.4 or BA.5, which the U.S. Food and Drug Administration (FDA) wants targeted in new boosters.

Moderna, in late June, said it should have a bivalent shot targeting those offshoots by early November. However, the “next generation” booster approved by the U.K. has stronger antibody responses to BA.5 than Moderna’s current booster, which has been used in the U.S. for nearly a year.

While Moderna is seeking authorization for mRNA-1273.214 in Australia, Canada, and the EU, it did not say it’s working with the FDA for approval. Some drug industry observers theorize that approving the booster that targeted BA.1, which was dominant in the U.S. in January, would dissuade pharma companies from expediting one that was focused on BA.4 and BA.5.

Omicron’s BA.5 subvariant has been deemed “the worst version” of the virus yet with high transmissibility, but the FDA’s request for boosters that target it has created some confusion among Americans over whether to get a booster when they’re eligible or wait for the updated one.

Doctors say the answer comes down to underlying health conditions. People at risk of severe COVID should get the existing vaccine as a second booster as soon as possible.