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 percent of the fully vaccinated U.S. population, according to the Centers for Disease Control and Prevention. More than 54 percent 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 amid concern about the omicron variant.

“Uncertainty and confusion is always going to translate into lower uptake,” said Alison M. 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.