Online comments are more influential on opinion on vaccinations than credible public service announcements (PSAs), according to a recent study.

Researchers at Washington State University found that with measles and other diseases once thought eradicated making a comeback in the United States, healthcare websites are on the spot to educate consumers about important health risks. The study comes after a recent outbreak of measles linked to Disneyland parks in California has affected at least 100 people in the United States and Mexico.

"In the context of health advertising, few issues have concerned advertisers, researchers and consumers -- especially those with young children -- more than recent trends in vaccination attitudes and behaviors," wrote researcher Loannis Kareklas and colleagues.

For the study, researchers conducted two experiments. In the first, they showed 129 participants two made-up PSAs.

Participants were led to believe that the pro-vaccination PSA was sponsored by the U.S. Centers for Disease Control and Prevention (CDC), while the anti-vaccination PSA was sponsored by the National Vaccine Information Council (NVIC). Both PSAs were designed to look like they appeared on each organization's respective website to enhance validity.

The PSAs were followed by comments from fictitious online commenters who either expressed pro- or anti-vaccination viewpoints. Participants weren't told anything about who the commenters were, and unisex names were used to avoid potential gender biases.

After looking at the PSAs and comments, people responded to questionnaires that rated their likelihood to vaccinate themselves and their family members, as well as their opinions about vaccination.

Results showed participants were equally persuaded by the PSAs and the online comments.

"That kind of blew us away," said Kareklas. "People were trusting the random online commenters just as much as the PSA itself."

In the second experiment, participants were told the fictitious commenters were an English literature student, a lobbyist specializing in healthcare issues and a medical doctor specializing in infectious diseases and vaccinology. The researchers determined that participants found the doctor's comments to be more impactful than the PSAs.

"We found that when both the sponsor of the PSA and the relevant expertise of the online commenters were identified, the impact of these comments on participants' attitudes and behavioral intentions was greater than the impact of the PSA and its associated credibility," the researchers wrote.

The researchers suggest that social advertisers must first be vigilant that their attempts to persuade are not perceived by readers as being manipulative or disingenuous. Health websites should include opposing viewpoints where relevant, but should also ensure that supportive comments are abundant, easily accessible and supported by research evidence.

"It would be advisable for some supportive comments from noted experts to be highlighted on health websites," they said.

 They recommended that advertisers clearly identify the expertise of the commenter -- for example, a medical doctor specializing in a related field of medicine.

The findings are detailed in the Journal of Advertising.