Officials and public health experts are strengthening their calls for people to get booster shots as the omicron variant proliferates across the United States, where less than a third of the fully vaccinated population has received an additional coronavirus vaccine dose.

As of Sunday, more than 60.2 million booster doses had been administered across the country — that’s enough for about 30% of the fully vaccinated U.S. population, according to the Centers for Disease Control and Prevention. More than 54% of vaccinated people 65 years and older have gotten a booster, the CDC said.

In public remarks in recent days, federal and state officials have implored people to get boosted as soon as they can. But public health experts and behavioral scientists say uncertainty about who needs boosters and how they help may explain why uptake isn’t higher. Still, they say booster numbers can rise, especially as messaging strengthens around the those doses amid concern about the omicron variant.

“Uncertainty and confusion is always going to translate into lower uptake,” said Alison Buttenheim, a behavioral scientist who studies vaccine acceptance at the University of Pennsylvania’s School of Nursing. “It’s pretty easy to say, ‘Oh I don’t need one, or I don’t know if I need one, I’ll wait until I get a clearer signal.’ “

In late November, federal regulators authorized Pfizer-BioNTech and Moderna coronavirus vaccine boosters for all adults. Before that expansion, federal guidelines said boosters were for those 65 and older, as well as for those at high risk because of underlying health conditions. Adults who had received the single dose Johnson & Johnson vaccine were eligible two months after vaccination.

The CDC says all adults 18 and older should get a booster, and 16- and 17-year-olds can also get them.

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The “relatively low” booster numbers can be also be explained by timing, said Wafaa El-Sadr, an epidemiology professor at Columbia University’s Mailman School of Public Health. The federal government only began recommending booster shots for everyone in recent weeks, she said.

As more data emerges about the value of boosters — particularly preliminary studies that suggest boosters offer significant protection against severe disease — that hopefully will compel people to get those shots, El-Sadr said.

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And for those who are still unsure, she said, public health officials should double their efforts to motivate and convey the message that “this is the moment to get it.”

Health experts stressed that there has been an overwhelming amount of information to keep up with on the coronavirus vaccines, and now on boosters — and in recent weeks that has been coupled with limited information about how well the vaccines and boosters protect against the latest variant.

“We should have been shouting about boosters from the rooftop for the last month, when we first essentially saw the variant was emerging,” said Rupali Limaye, a behavioral and social scientist who studies vaccine hesitancy at the Johns Hopkins Bloomberg School of Public Health. She said there “has to be more urgency and recommendations that we need to get this and we need to get this ASAP.”

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Limaye said those recommendations need to come not just from the White House but from leaders on both sides of the aisle.

In recent days, as infections have surged in cities including New York and Los Angeles, the rhetoric appears to be accelerating. In a White House briefing on Friday, Anthony Fauci, the nation’s leading infectious-disease specialist, said “fully vaccinated plus a boost” offers the best protection against COVID-19, the disease the coronavirus causes.

“It is critical to get vaccinated. If you are vaccinated, it is critical for optimal protection to get boosted,” Fauci said.

He warned Sunday that the nation could see record coronavirus cases as the variant spreads, days after President Joe Biden said that for the unvaccinated, “we are looking at a winter of severe illness and death.”

Francis Collins, director of the National Institutes of Health, warned people not to wait to get an extra shot.

“Get a booster,” he said in a Sunday interview on CBS’s “Face the Nation.” “This is the week to do it. Do not wait.”

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When attention is a “limited resource,” Buttenheim said it helps when messaging from officials and experts aligns with what people are experiencing around them.

“Everyone is saying this feels like March 2020 or December 2020 again,” she said, noting New York City, where events are shuttering and anxiety is rising as the spread of omicron upends holiday plans.

“If you hear those things and you see those things and there’s holiday travel and people are stressed, that’s good fertile ground for a more urgent recommendation for boosters from the White House or your physician to resonate,” Buttenheim said. It signals to people that there’s “some urgency and importance around this.”

Still, she stressed that people are in different “info and social bubbles,” and that public health messaging may not be resonating with everyone in the same way.

As the public health crisis wears on, Buttenheim underlined the fatigue everyone is feeling.

“It’s just getting harder and harder to make a clear nationwide recommendation on something and hope that people are hearing it, she said, adding: “We haven’t done a good job of shifting from urgent, emergency-level, to what’s clearly going to be a few more years ahead of managing ebbs and flows.”

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Adding to that confusion, El-Sadr said, is that officials have not been clear enough about the limitations of vaccines.

“People then started hearing that their fully-vaccinated friend was infected and thought ‘this must mean vaccines don’t really work'” the epidemiology professor said. “A clearer message about vaccines not eliminating risk of infection but still provide significant level of protection would have helped.”

For Delaney Williams, a pharmacist in Seattle, a booster was a resource against the virus and anxiety. She got her booster in October, when delta variant cases were surging.

“It gave me a lot of peace of mind,” she said. “It felt like a weight was lifted.”

Williams and her husband, who was vaccinated and scheduled to get his booster, were recently exposed to the coronavirus after a weekend trip. He got infected. She didn’t, even though they were not able to isolate apart from each other.

“It really is like a no-brainer to me, and I see no downsides to it and only positive things,” she said. “You get to protect yourself, your loved ones and your community.”

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The Washington Post’s Dan Keating contributed to this report.