The United Kingdom has become the first country to approve vaccine boosters designed to target the omicron variant of COVID-19, paving the way for Brits to receive their shots in early fall.
The Moderna shot approved in the U.K. is "bivalent," meaning it's a mix of two versions of the vaccine: Half is targeted at the original strain of COVID-19, and half is a new formulation designed to fight the original omicron variant, also known as BA.1.
"What this bivalent vaccine gives us is a sharpened tool in our armoury to help protect us against this disease as the virus continues to evolve," Dr. June Raine, the head of the U.K.'s Medicines and Healthcare Products Regulatory Agency, said in a statement.
Researchers in the U.K. found that the Moderna-made omicron booster "triggers a strong immune response" against both the original 2020 strain of the coronavirus and the original omicron variant, which emerged late last year.
What's been approved in the U.K. is a different shot than the omicron-specific booster that U.S. officials hope to release this fall.
Here in the U.S., the FDA has asked vaccine developers to target the omicron subvariants known as BA.4 and BA.5 — the two strains that currently make up the vast majority of cases here — rather than focus on the original omicron variant, which swept across the country last winter.
The Moderna shot approved Monday in the U.K. was less effective against BA.5 — though it still "generate[d] a good immune response" against that strain, researchers said.
British regulators also found that the side effects of the bivalent vaccine were similar to that of the original Moderna vaccine – "typically mild and self-resolving" – and that there were no serious safety concerns.
Why does the U.S. want different omicron boosters?
But these bivalent shots were designed around the original omicron variant, and as a result, they're less effective against BA.4 and BA.5.
The original omicron variant emerged in late 2021 and helped drive the massive wave of infections during last year's holiday season. It has since mutated into a handful of different subvariants that, one by one, came to dominate the caseload. Now, the BA.5 subvariant is dominant in the U.S., comprising nearly 90% of all cases.
That rapid shift in the makeup of the virus has been "a moving target" for health officials trying to direct vaccine policy, said Dr. Anthony Fauci, the top infectious disease official in the U.S.
In late June, the FDA, with its eyes on the likelihood of another winter surge, decided to ask vaccine manufacturers to create a bivalent vaccine that targeted BA.4 and BA.5, rather than the original omicron strain.
"Hopefully, that will be close enough to whatever variant evolves as we get into the fall and into the winter," Fauci said in an interview with NPR late last month.
When will the omicron boosters be available in the U.S.?
Officials have signaled that omicron-specific boosters will be available to Americans sometime this fall.
The U.S. has purchased more than 170 million total doses of omicron boosters from Pfizer and Moderna. (That's not enough for all 330 million Americans. But only about two-thirds of Americans finished their initial course of the vaccine, and fewer than half of those have received booster shots.)
The regulatory process that helps show the vaccines are safe and effective is still underway. Officials have said they hope to authorize the boosters by mid-September.
Can I just get another original booster now?
Americans who are 50 years old or older, along with some immunocompromised people, can already get a second booster shot, according to CDC guidelines.
For others, some experts say that the new boosters specific to BA.4 and BA.5 are expected soon enough that it's worth waiting.
Most people under 50 who don't otherwise have underlying conditions already have some protection. Even though the effectiveness of the original vaccine and booster have waned some, they still help, especially at preventing serious infections and hospitalization. In addition, some estimates say as many as 80% of Americans have caught COVID-19, adding natural immunity to the mix.
Plus, vaccines may be less effective when taken too close together. Taken altogether, if you're young and otherwise healthy, it may be best to wait for the new booster this fall, said Dr. Carlos del Rio, an infectious disease specialist at Emory University.
"If you get a vaccine right now, the concern is that you will not respond as well when you get another vaccine so close to this one. You have to have some time between doses of vaccines," del Rio said in an interview with NPR earlier this month. "In other words, there's more risk than benefit of getting another booster right now."