Pelé, Dolly Parton and the Dalai Lama have little in common apart from this: Over a few days in March, they became the latest celebrity case studies for the health benefits of Covid-19 vaccines.

“I just want to say to all of you cowards out there: Don’t be such a chicken squat,” Ms. Parton, 75, said in a video that she posted on Twitter after receiving her vaccine in Tennessee. “Get out there and get your shot.”

This is hardly the first time public figures have thrown their popularity behind an effort to change the behavior of ordinary people. In medicine, celebrity endorsements tend to echo or reinforce messages that health authorities are trying to publicize, whether it’s getting a vaccine, or other medical treatment. In 18th-century Russia, Catherine the Great was inoculated against smallpox as part of her campaign to promote the nationwide rollout of the procedure. Almost 200 years later, backstage at “The Ed Sullivan Show,” Elvis Presley received the polio vaccine in an effort to help reach at-risk teenagers.

But do the star-studded endorsements really work? Not necessarily. Epidemiologists say there are plenty of caveats and potential pitfalls — and little scientific evidence to prove that the endorsements actually boost vaccine uptake.

“Very few people actually do give the weight of expertise, for better or worse, to celebrities,” said René F. Najera, an epidemiologist and the editor of the History of Vaccines website, a project of the College of Physicians of Philadelphia.

“There’s some shift there now with social media and social influence in the younger age groups,” he added. “But for the most part, we still listen more to our peers than to some figurehead.”

As vaccination campaigns accelerate around the world, watching high-profile endorsements has become one of the latest — and among the weirdest — online rituals of the Covid era.

To help track the phenomenon, New York Magazine over the winter kept a running list of newly vaccinated celebrities that includes Christie Brinkley (“piece of cake”), Whoopi Goldberg (“I didn’t feel it”) and Mandy Patinkin (“One of the few benefits of being old”). Journalists in India have done the same for Bollywood film stars.

In Europe, pictures of male politicians getting their shots while shirtless have generated a bunch of memes. An epidemiologist in Oregon, Dr. Esther Choo, joked on Twitter that the French health minister, Olivier Véran, was carrying out a public-relations campaign that she called “Operation Smolder.”

Such posts are notable because they instantly allow millions of people to see the raw mechanics of immunization — needles and all — at a time when skepticism toward Covid vaccines has been 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.

There is some evidence that celebrity endorsements of a given medical behavior can have concrete results. After Katie Couric 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.

And in Indonesia, researchers found in a pre-coronavirus 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.

Dr. Najera and other researchers have been convening focus groups of Americans to find out what has prompted them to agree — or not — to be vaccinated against Covid-19. He said the primary finding so far was that rates of uptake or hesitancy often corresponded to vaccine behavior among a given person’s racial, ethnic or socioeconomic peer group.

Ho Phi Huynh, a professor of psychology at Texas A&M University-San Antonio, said that vaccine endorsements from celebrities tended to have a “spectrum of effect” because the degree of star admiration varies so much from fan to fan. Some see a celebrity merely as entertainment, Dr. Huynh said, while others form attachments to them that may compensate for a lack of authentic relationships in their own lives.

“So going back to Dolly, if people perceive her to be a ‘typical liberal’ celebrity, there might be little influence for a large faction of the country,” he said.

In Indonesia this winter, it took only a few hours for a mega-celebrity to undercut his own vaccine endorsement.

The government had chosen the entertainer Raffi Ahmad, 34, to be 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.”