NEW YORK — The COVID-19 booster drive in the U.S. is losing steam, worrying health experts who have pleaded with Americans to get an extra shot to shore up their protection against the highly contagious omicron variant.

Just 40% of fully vaccinated Americans have received a booster dose, according to the Centers for Disease Control and Prevention. And the average number of booster shots dispensed per day in the U.S. has plummeted from a peak of 1 million in early December to about 490,000 as of last week.

Also, a new poll from The Associated Press-NORC Center for Public Affairs Research found that Americans are more likely to see the initial vaccinations — rather than a booster — as essential.

“It’s clear that the booster effort is falling short,” said Jason Schwartz, a vaccine policy expert at Yale University.

Overall, the U.S. vaccination campaign has been sluggish. More than 13 months after it began, just 63% of Americans, or 210 million people, are fully vaccinated with the initial rounds of shots. Mandates that could raise those numbers have been hobbled by legal challenges.

Vaccination numbers are stagnant in states such as Wyoming, Idaho, Mississippi and Alabama, which have been hovering below 50%.

In Wyoming, 44% are fully vaccinated, up just slightly from 41% in September. To boost numbers, the state has been running TV ads with health care workers giving grim accounts of unvaccinated people struggling with COVID-19.

And in neighboring Idaho, which also has one of the country’s lowest vaccination rates, the number of people getting their first vaccine dose has remained under 1,000 almost every day this year and the number getting booster shots is also declining.

“I don’t like to use the word ‘resigned,’” said Elke Shaw-Tulloch, administrator of the Idaho Division of Public Health. “I think we just need to keep saying it over and over again, how important it is.”

At the other end of the spectrum, Vermont is a national leader in the percentage of people who have been fully vaccinated and received a booster shot. About 60% of the population over 18 has gotten a booster.

The U.S. and many other nations have been urging adults to get boosters because the vaccine’s protection can wane. Also, research has shown that while the vaccines have proved less effective against omicron, boosters can rev up the body’s defenses against the threat.

As for why an estimated 86 million Americans who have been fully vaccinated and are eligible for a booster have not yet gotten one, Schwartz said public confusion is one important reason.

“I think the evidence is now overwhelming that the booster is not simply an optional supplement, but it is a foundational part of protection,” he said. “But clearly that message has been lost.”