The U.S. is poised to make COVID-19 vaccinations more like a yearly flu shot, a major shift in strategy despite a long list of questions about how to best protect against a still rapidly mutating virus.

The Food and Drug Administration asked its scientific advisers Thursday to help lay the groundwork for switching to once-a-year boosters for most Americans, and how and when to periodically update the shots' recipe.

"This is a consequential meeting to determine if we've reached the point in the pandemic that allows for simplifying the use of current COVID-19 vaccines," said FDA's vaccine review head Dr. David Kaslow.

The advisory panel mostly agreed with the FDA's approach.

COVID-19 vaccines have provided certain protections to the most vulnerable, but protection does wane and the shots don't fend off milder infections for long. And Americans are tired of getting vaccinated. 

While more than 80 percent of the U.S. population has had at least one COVID-19 shot, only 16 percent of those eligible for the latest bivalent boosters have gotten one.

That calls into question the decisions made on who should receive an additional shot, how frequently and what kind.

The FDA advisory panel voted unanimously that people should get the same vaccine formula whether they're receiving their initial vaccinations or a booster. Today, Americans get one formula based on the original coronavirus strain that emerged in 2020 for their first two or three doses – and their latest booster is a combination shot made by Pfizer or Moderna that adds omicron protection.

The FDA would have to decide how to phase in that change.

Since the original coronavirus strain has disappeared, "moving towards the strains that are circulating is very important," said Dr. Archana Chatterjee, dean of Chicago Medical School.

Looking ahead, the FDA said most Americans should do fine if they get a once-a-year booster targeted to the newest variants in the fall, but adults with weakened immune systems and very young children who've never been previously vaccinated might need two – similar to how youngsters get their first-ever flu vaccination.

However, it is not clear if younger, healthier people would need a COVID-19 booster every year.

The vaccination period is something that would depend on when infections start rising and how long a booster's protection might last, said FDA adviser Dr. Arthur Reingold of the University of California, Berkeley.

Reingold added that the autumn might not even be the best time to boost.

Unlike flu which in the U.S. circulates mostly during late fall and winter, COVID-19 waves have occurred year-round.

As for the recipe, the FDA plans to call its advisory panel for another meeting in late May or early June to decide if the vaccine recipe needs tweaking, including which virus strain to target and whether it should be a single-strain or multi-strain shot. 

Pfizer and Moderna said that would give enough time to produce needed doses by fall while a third manufacturer, Novavax, urged an earlier start to any recipe change.

(With input from AP)