(CNN)When celebrities speak, it seems, the world listens -- even when it comes to personal and public health.
Just look at what happened after Charlie Sheen's HIV disclosure two years ago, said John Ayers, a research professor at San Diego State University.
Sales of in-home HIV testing kits reached record highs around the same time the actor announced that he had been diagnosed as HIV-positive in 2015, according to a study published in the journal Prevention Science on Thursday. Ayers was a co-author of the study.
About the same time, "in record numbers, people were going online, seeking out information on what the signs of HIV are, on how to find and appropriately prevent HIV with devices such as condoms and also how to get tested,'" Ayers said. "We've seen this ... many times over."
For the new study, Ayers and his co-authors monitored weekly sales of OraQuick, the only FDA-approved at-home oral HIV testing kit available in the United States, from 2014 to 2016.
The researchers found that there were 8,225 more sales than they expected the week of Sheen's HIV status announcement. Elevated sales continued for four weeks after Sheen's disclosure.
The findings show a correlation, not a direct causation. Yet in a previous study, Ayers and his colleagues also found that Google searches for HIV testing and related topics also spiked after Sheen's announcement.
"The most common reaction is, 'So what? What does a search really mean?' Our new study shows not only did Sheen's disclosure lead people to seek information about HIV prevention, it also corresponded with record levels of at-home rapid HIV testing sales," Ayers said.
Though they are not doctors, many celebrities have had both positive and negative ties to public health in recent years.
"The positive example I often point to is when Magic Johnson talked about being HIV-positive, which provided a moment to bust stigma around HIV," said Steven Hoffman, scientific director of the Institute of Population and Public Health in Canada and director of the University of Ottawa's Global Strategy Lab.
In 1991, when most people didn't have much information about the virus and the Internet was not commonly used, the American basketball star announced that he had tested positive for HIV and retired from the NBA.
"Here was a very prominent athlete who is publicly disclosing being HIV-positive and opening up conversation around it for people," said Hoffman, who was not involved in the latest study. "I think that's an example highlighting the positive role that celebrity can play in informing health decisions."
Yet while Johnson's HIV disclosure spurred questions and conversations, since the virus was still unfamiliar, Sheen's disclosure was linked to action, Ayers said.
After Sheen's announcement, about 1.25 million people in the United States searched online for topics directly related to condoms, HIV symptoms or HIV testing, according to Ayers' previous study, published in the journal JAMA Internal Medicine last year.
That study involved analyzing Google Trends and search term data from 2004 through November 2015, which included when Sheen disclosed his HIV status on November 17, 2015.
The researchers found that Sheen's announcement corresponded with the greatest number of HIV-related Google searchers ever recorded in the US. But Sheen didn't make an immediate call to action for the public to get tested or use condoms, and public health groups didn't use him as a face to raise HIV awareness.
Therefore, Sheen's disclosure was an example of how -- without the celebrity or public health leaders calling for action -- the public appeared to still respond based on health information that was widely known, Ayers said.
"When Magic Johnson made his disclosure, there wasn't a similar reaction. There couldn't be, because people didn't know what to do," Ayers said.
"Remember, people (were) asking, 'Can he play basketball?' 'Can I shake his hand?' There wasn't this trove of information about HIV, HIV prevention and HIV testing. When Sheen disclosed, there was this trove of information," he said. "We saw people seeking out information on the signs of HIV, HIV testing and how to prevent HIV by using condoms. We saw this trilogy of reaction that is fairly consistent with what public health has been promoting for 20-plus years."
Kami Kosenko, an associate professor of health communication at North Carolina State University, said that she believes actress Angelina Jolie's 2013 announcement of a double mastectomy to prevent breast cancer also had a positive influence on public health, despite possibly influencing an uptick in unnecessary genetic testing.
Jolie said she made her decision because she has a mutation in her BRCA1 gene. Specific inherited mutations in BRCA1 increase the risk of breast and ovarian cancers, and Jolie's mother died of ovarian cancer. Jolie decided to also have her ovaries and fallopian tubes removed in 2015.
Kosenko led a study in which 229 women were surveyed immediately after Jolie's announcement to undergo a double mastectomy. Researchers looked at whether Jolie influenced the women's genetic testing intentions.
The study, published in the Journal of Health Communication last year, showed that 30% of the women intended to get tested to see whether they carried the BRCA1 gene, 23% said they would probably get tested, and 7% said they would definitely get tested. The others reported that they would not get tested.
Kosenko said that separate studies demonstrated increased genetic testing requests and Internet searches related to BRCA1 after Jolie's announcement.
"We rely heavily on friends and family for health information, and we tend to see certain celebrities as friends. Unfortunately, we do not share the same resources as these famous 'friends,' which impacts our ability to follow celebrity health advice," she said. "For example, at the time of Jolie's announcement, the genetic tests she underwent were prohibitively expensive for the average American."
There are examples of celebrities having a potentially harmful influence on public health too, said Hoffman, director of the University of Ottawa's Global Strategy Lab.
"The most prominent example of harm would be, in my mind, Jenny McCarthy," Hoffman said.
The American actress has been a vocal autism activist, spreading concerns that autism might be associated with childhood vaccines.
McCarthy was unavailable for comment, but her representative said she addressed her stance on vaccines in an op-ed published in the Chicago Sun-Times in 2014, headlined "The gray area on vaccines."
Her son, Evan, was diagnosed with autism. She believes that doctors should change some children's vaccine schedules, such as allowing for one shot per doctor's visit instead of multiple shots, she wrote in the op-ed.
"I am not 'anti-vaccine.' This is not a change in my stance nor is it a new position that I have recently adopted. For years, I have repeatedly stated that I am, in fact, 'pro-vaccine' and for years I have been wrongly branded as 'anti-vaccine,' " McCarthy wrote.
"My beautiful son, Evan, inspired this mother to question the 'one size fits all' philosophy of the recommended vaccine schedule," she wrote. "I embarked on this quest not only for myself and my family, but for countless parents who shared my desire for knowledge that could lead to options and alternate schedules, but never to eliminate the vaccines."
"She is actually recommending practices that are unhealthful and potentially dangerous," Hoffman said about McCarthy's stance on changing vaccine schedules.
"Recommending to people not to vaccinate their children means not only are the children being subjected to unnecessary harm, but many others around those children who might not be able to get vaccinated for legitimate reasons are put at an additional risk ask well," he said. "It's not only unfair to the children who aren't being vaccinated, but also to the many people who can't be vaccinated, who interact with those children."
Even though they are vulnerable to illness, some people may not be able to get certain vaccines based on age, an allergy or pre-existing health condition, or other factors, according to the CDC.
Public health experts also have accused actress Gwyneth Paltrow of spreading some questionable health tips through her lifestyle blog, goop.
In a written statement, goop spokeswoman Noora Raj Brown described the website as pushing "new ideas into the conversation" in health and wellness.
"We have a disclaimer on the bottom of every article that states the purpose is to highlight alternative studies and induce conversation. It's not a substitute for professional medical advice, diagnosis, or treatment. The views of the experts profiled don't necessarily represent the views of goop," the statement said.
One blog post on goop recommended inserting a rock, called a jade egg, into your vagina to improve your sex life and orgasms, among other benefits.
Some experts have said that not thoroughly cleaning or sterilizing the jade egg could lead to infection, or that it could get stuck.
"In many cases, there are opposing opinions on issues -- like the jade eggs for example -- and our role is to service the conversation," Brown said.
"Gwyneth Paltrow's messages, it's mostly that she's recommending practices that don't work," Hoffman said. "The biggest harm is the loss or waste of money and the misplaced attention away from practices that actually work."
A jade egg on goop costs about $66. Hoffman said that the money could be better spent on buying nutritious foods or a gym membership.
"Do celebrities have an impact on health outcomes? We know they do. ... There's so many studies out there that have demonstrated that when celebrities either give helpful or harmful advice to people, it's routinely followed," Hoffman said.
"I do blame celebrities, because they need to recognize the impact they can have and the potential harm that can result from it," he said. "I definitely blame those who are spreading misinformation."
Hoffman and his colleagues published a review paper in the British Medical Journal to analyze the influence celebrities have on people's health decisions through the factors of economics, marketing, psychology and sociology.
"In marketing literature, they talk about a halo effect around celebrities, whereby people have a very positive view of celebrities, and when celebrities endorse a product or recommend a particular health practice, it transfers their golden glow from the celebrity to the products or practice," Hoffman said.
"In economics literature, there's recognition of something called signaling, which is whereby in a marketplace filled with competing ideas, people are constantly looking for a shortcut in order to identify which products or practices they should be following, and so a celebrity endorsing a product provides a cognitive shortcut to an otherwise very complicated decision," he said. "There is increasing recognition of the role of social networks, and there's few people who have bigger social networks than celebrities. Just think of the number of Twitter followers some celebrities have cultivated."
However, Ayers, author of the latest study, said it might not be that complicated.
"It's not so much about a celebrity effect," he said, adding that when celebrities speak out about health-related issues, it may resonate because they are speaking as humans with just as much vulnerability to health impacts as anyone else.
"Typically, public health uses top-down strategies. The most effective messages are believed to come from the experts, 'from the top.' The people who know the most about the subject, the people who are paid to sit around and think about the subject," Ayers said.
"It turns out, though, that if you look at the Sheen effect and if you look at these other examples of organic media events ... that's not the case. These messages, they come from the bottom; they come from ordinary people. People who aren't health experts. Sheen might be a celebrity, but he's not a health expert. Angelina Jolie is not a health expert," he said.
All in all, Hoffman said, public health officials could work with celebrities to harness their fame to spread evidence-based health information and mitigate the spread of misinformation.
He cited supermodel Christy Turlington Burns as a positive example of a celebrity working with public health authorities to promote positive health practices. She served as a spokeswoman in CDC promotional ads raising awareness about smoking cessation and the health risks of smoking.
Ayers said he saw public health officials utilize the power of Sheen's celebrity in real time, not during but after his HIV announcement.
"It turns out that actually because of our study, we got to see those things happen. When we published our study, a few weeks later, Charlie Sheen began speaking out about HIV prevention, particularly condoms, and he cited our study as one of the reasons for doing that," Ayers said of the study on Google searches. "We also saw several leading organizations in HIV prevention begin to use Charlie Sheen's disclosure as a way to promote HIV prevention."
Hoffman said it would be beneficial to see more examples of celebrities and public health leaders working together in the future.
"I think there probably are a lot of celebrities who want to do good and would be open to the idea of working with public health officials to do good," he said. "We just need to still figure out the best way of doing that, in terms of what would be most effective."
For now, to make informed health decisions, Dr. Georges Benjamin, executive director of the American Public Health Association, offered some guidance in an email.
"Get a second opinion," he wrote. "Trust but verify celebrity statements from trusted health sources like the American Cancer Society, the Center for Disease Control and Prevention, APHA, American Academy of Pediatrics" and others.