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.