"I have anxiety. And I’m treating my anxiety. I’m on medication for my anxiety. Why am I saying this? Because it’s a completely normal thing. It’s a biological problem. In my family there have been cases of depression. More and more people should talk openly about it. There is nothing shameful about it or something to hide. If you had a constant stomach pain, wouldn’t you go to the doctor? It’s that simple. I want to make this my mission, to take any shame out of this, to educate people about this."
Roughly around the same time another Indian actress Deepika Padukone came out openly about her depression in an interview with a national newspaper.  She talked about her plans to create more awareness about depression and also used social media for the same.
"Anxiety,Depression and Panic Attacks are not signs of weakness.They are signs of trying to remain strong for way too long." - @deepikapadukone, 31 Dec, 2014 
They might not be the first to open up about this in India but their words did not go unnoticed. It triggered a sensible discussion and reasonably positive media coverage. Few more celebrities came out and talked about their issues. A good message was reaching the public that anxiety/depression are not the end of the world and it is perfectly fine to talk about it or reach out for help. Talking about mental issues always appeared like a huge taboo in India. So these incidents and follow-up discussion seemed like a very welcomed change. I think for the same impact the government would have required a lot more resources. Imagine a senior mental health researcher from AIIMS stating the same guidelines in an interview. I think you would agree, though the researcher is well-qualified, the reach/impact would have been a lot less. So subject matter expertise does not necessarily assure reach/impact to the target audience. On the other hand, a couple of celebrity interviews had done the trick.
In 2014, CNN published an article - India beats the odds, beats the Polio. 
"In 2009, India still reported half of the world's new cases -- 741 out of 1,604. India has millions of poor and uneducated people. The population is booming. Large areas lack hygiene and good sanitation, and polio spreads through contaminated water. Many health experts predicted India would be the last country in the world to get rid of polio. They were wrong."
It was a great teamwork. The WHO, the government and so many other contributors played their part. But do you remember how you used to know about a next planned vaccination drive? For the most part through Amitabh Bachchan, the superstar of Indian cinema. He appeared in TV and radio ads, billboards and wherever he could with the message to vaccinate your children. In 2014 India was declared a Polio-free country. Later that year Amitabh Bachchan was awarded for his contribution by Union Health minister and Unicef India representative. The Hindu published an article appreciating his efforts with the title "When Amitabh's voice did the trick to make India polio-free."  After this success, the actor seems to be gearing up for another campaign related to Hepatitis B. Wonderful way to use your charisma!
We can see similar pattern in developed countries as well. In 2014, Angelina Jolie wrote in The New York Times about her mother's death due to breast cancer and her own high-risk situation.  In this article, she specifically mentioned the risk associated with BRCA genes and hoped many other women would test themselves to learn about risk level. This single article in NYT appears to have created a significant impact. The BMJ (old British Medical Journal) reported a spike in genetic tests related to the gene (BRCA) associated with increased risk of Breast Cancer.  The conclusion of this observational study states, "Celebrity endorsements can have a large and immediate effect on the use of health services. Such announcements can be a low-cost means of reaching a broad audience quickly, but they may not effectively target the subpopulations that are most at risk for the relevant underlying condition."
A similar incident happened in Australia. A pop singer, Kylie Minogue was diagnosed with breast cancer in 2005. The Medical Journal of Australia followed this case and reported, "News coverage of Kylie Minogue’s breast cancer diagnosis caused an unprecedented increase in bookings for mammography". Additionally, there was a 20-fold increase in news coverage of breast cancer, which emphasized that young women do get breast cancer and that early detection was critical. 
However, this celebrity influence in public health seems to be a double-edged sword. In 2014, Indian actress Madhuri Dixit advertised the Maggi instant noodles claiming it is very healthy. Months after the campaign was launched by the company, Uttar Pradesh FDA found these noodle packets containing MSG and lead more than the permissible limit. To make the matter worse the courts in Muzaffarpur and Barabanki ordered FIRs against celebrities for endorsing Maggi. In addition to Madhuri Dixit, it included the same Amitabh Bachchan we thanked for Polio campaign. 
A researcher in 1998 came up with fraudulent paper (which was later retracted) that appeared to have linked MMR vaccine and autism. Things blew out of the proportion, it became the biggest science story in 2002. The fear spread throughout the country and eventually Tony Blair was asked if his infant son had the MMR jab. Mr. Blair who had supported the MMR program refused to state if his son is vaccinated. Some of us might agree with Tony Blair on the grounds of privacy. However, the point is about the message that was sent to already scared parents and things that followed. Sir Liam Donaldson, who was Chief Medical Officer of England during that period have criticized Tony Blair for not going public with his son being vaccinated. I guess when you hold a public office, your clear stand on public health issues is more important than you privacy concerns.
The rising consumption of sugary drink is a major contributor to the Obesity epidemic. These drinks which contain almost no nutritional value but sugar. France recently banned the unlimited refills of these drinks as part of the battle against obesity. However, we constantly see celebrities from Britney Spears to Justin Timberlake endorsing these products. In fact, these soft drink companies appear to have advertisement contracts with major celebrities from almost all countries.
How these celebrity opinions and endorsements affect the choice of people is a fascinating area.
Two years ago, researchers from US and Canada looked into this by studying existing research papers available to this topic (like a study of existing studies).  The research was aimed at how celebrity engagements can benefit or hinder efforts to educate patients on evidence-based practices and improve their health literacy. The result section of this paper says, "According to the economics literature, celebrities distinguish endorsed items from competitors and can catalyze herd behavior. Neuroscience research supports these explanations, finding that celebrity endorsements activate brain regions involved in making positive associations, building trust and encoding memories. The psychology literature tells us that celebrity advice conditions people to react positively toward it."
It gets even more fascinating. There is a follow-up project which considered even more research literature with a significantly expanded team. This time researchers aimed to get answers to more specific questions like,
-- Which health-related outcomes are influenced by celebrities?
-- How large of an impact do celebrities actually have on these health-related outcomes?
-- Under what circumstances do celebrities produce either beneficial or harmful impact?
They hope that the results of this will contribute to the understanding of celebrity influences and further help to design positive evidence-based celebrity health promotion activities. In addition, these findings can help inform the development of media reporting guidelines pertaining to celebrity health news.
If we can get reasonably accurate answers to these questions, it would be immensely helpful. We could choose celebrities for the promotion of health initiatives in order to maximize the impact. At the same time, there will be some risk of bad companies using this science for monetary benefits and against the public interest. Let's be positive and hope celebrities will understand the power of influencing public health they have. After all as one of them said, "With Great Power Comes Great Responsibility".