stubbornly persistent in the United States and beyond. The rapid-fire testimonials by Pelé, Ms. Parton and the Dalai Lama in March, for example, collectively reached more than 30 million followers and prompted hundreds of thousands of engagements across Twitter, Instagram and YouTube. In April, the singer Ciara hosted a star-studded NBC special meant to promote vaccinations, with appearances by former President Barack Obama and his wife, Michelle Obama, as well as Lin-Manuel Miranda, Jennifer Hudson, Matthew McConaughey and others.

“These kind of endorsements might be especially important if trust in government/official sources is quite low,” Tracy Epton, a psychologist at the University of Manchester in Britain who has studied public health interventions during the coronavirus pandemic, said in an email.

That was the case in the 1950s, when Elvis Presley agreed to receive the polio vaccine to help the National Foundation for Infantile Paralysis reach a demographic — teenagers — that was “difficult to educate and inspire through traditional means,” said Stephen E. Mawdsley, a lecturer in modern American history at the University of Bristol in Britain.

“I think Elvis helped to make getting vaccinated seem ‘cool’ and not just the responsible thing to do,” Dr. Mawdsley said.

had a colonoscopy live on the “Today” show in 2000, for example, the number of colorectal screenings in the United States soared for about nine months.

experiment that when 46 celebrities agreed to tweet or retweet pro-immunization messages, their posts were more popular than similar ones from noncelebrities. That was especially true when the celebrities delivered the message in their own voices, rather than citing someone else, researchers found.

“Their voice matters,” said Vivi Alatas, an economist in Indonesia and a co-author of that study. “It’s not just their ability to reach followers.”

For the most part, though, the science linking celebrity endorsements to behavioral change is tenuous.

One reason is that people generally consider those within their own personal networks, not celebrities, the best sources of advice about changing their own behavior, Dr. Najera said.

He cited a 2018 study that found few gun owners in the United States rated celebrities as effective communicators about safe gun storage. The owners were far more likely to trust law enforcement officers, active-duty military personnel, hunting or outdoor groups, and family members.

among the first in the country to receive a Covid shot in January. “Don’t be afraid of vaccines,” he told his Instagram followers, who numbered nearly 50 million at the time, almost a fifth of the country’s population.

That night, he was spotted partying without a mask, and accused of breaking the public’s trust.

“Please you can do better than this,” Sinna Sherina Munaf, an Indonesian musician, told Mr. Ahmad and her nearly 11 million followers on Twitter. “Your followers are counting on you.”

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