“I just want to say to all of you cowards out there: Don’t be such a chicken squat,” Parton (inset), 75, said in a video 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 publicise, 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 immunisation — 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, 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.