AP Medical Writer

Scientists have found clues that the world’s leading COVID-19 vaccines offer lasting protection that could diminish the need for frequent booster shots, but they caution that more research is needed and that virus mutations are still a wild card.

Critical studies are underway, and evidence is mounting that immunity from the mRNA vaccines made by Pfizer and Moderna does not depend exclusively on antibodies that dwindle over time. The body has overlapping layers of protection that offer backup.

Pfizer and Moderna have fueled booster questions by estimating that people might need yearly shots, just like with flu vaccinations, and the companies are working to have some candidates ready this fall. But companies will not decide when boosters get used. That will be up to health authorities in each country.

Other experts say boosters may be needed only every few years.

“I would be surprised if we actually needed a yearly booster shot,” said Dr. Paul Offit, a vaccine specialist at the Children’s Hospital of Philadelphia who advises the Food and Drug Administration.

They point to ways the immune system remembers the coronavirus so that once original antibodies fade, the body’s defenses can swing back into action if a person is exposed again.

“I’m pretty optimistic. I wouldn’t rule out the need for boosters, but the immune response so far looks actually quite impressive,” University of Pennsylvania immunologist John Wherry said.

Antibodies that form after vaccination or natural infection do wane naturally, but there’s evidence that those levels remain strong for at least six to nine months after mRNA vaccination and possibly longer. They also appear effective against worrisome virus mutants, at least for now.

Scientists do not yet know what’s called the correlate of protection, the level below which antibodies cannot fend off the coronavirus without additional help.

Dr. Anthony Fauci, the U.S. government’s leading infectious disease expert, told a Senate subcommittee last week that vaccine protection would not be infinite.

“I would imagine we will need, at some time, a booster,” Fauci said. “What we’re figuring out right now is what that interval is going to be.”

To date, 62.8% of the adult U.S. population has received at least one dose of a COVID-19 vaccine and 133.6 million, or more than 40 percent, are full vaccinated. The rate of new vaccinations has slowed to an average below 600,000 per day, according to the Centers for Disease Control and Prevention. That’s closing in on President Joe Biden’s goal of 70% with at least one inoculation by July 4. Infections and deaths continue to fall. The nation’s seven-day average for daily new cases fell to less than 17,300 on Tuesday, down from more than 31,000 two weeks ago. Daily deaths declined to 588, down from 605, according to data from Johns Hopkins University.