In the wake of the recent measles outbreak in California and with the threat of more to come, it's clear that existing efforts to encourage vaccination and promote public health aren't enough. We need to understand why the message isn’t sinking in where it’s most needed—why people believe what they believe and how they discover information they trust.

A recent paper in the Journal of Advertising suggests that online commenting may sway people as much as public service announcements (PSAs) from health authorities. Depending on who's doing the commenting, comments may sometimes be even more influential than PSAs.

While it may seem ludicrous that people could trust online comments as much as PSAs, it’s important to remember that online comments have an important advantage: they’re seen as coming from an unbiased source. This is why product reviews by consumers online and other “electronic word-of-mouth” communications are often seen as a trusted resource.

The question is whether this trust extends to matters of public health, where messages from public health organizations surely carry significant weight due to the expertise behind them. Expertise does play a role in how much people trust messaging, while it’s hard to ascertain the credibility of online commenters.

Credibility beats anonymity

To test how much people trusted online commenters compared to PSAs, the authors of the paper assigned 219 adults (recruited from Amazon’s Mechanical Turk) to view either a pro-vaccination or anti-vaccination PSA. The participants were led to believe that the pro-vax message was sponsored by the Centers for Disease Control and Prevention (CDC) and the anti-vax message sponsored by the National Vaccine Information Center (NVIC). A pretest confirmed that these organizations were seen as equal in credibility.

Each PSA (designed to look as though it appeared on a website) was followed by four online comments, either agreeing or disagreeing with the PSA. The commenters were provided with gender-neutral names to avoid possible bias. After reading this information, participants were surveyed to find out their attitudes toward vaccinating themselves and their families and whether they intended to get themselves or their families vaccinated. They were also asked to rate the credibility of both the PSAs and commenters.

All participants were debriefed with accurate information after the study was completed, so hopefully no longterm damage to vaccination efforts were done. Not all the participants were able to correctly state whether the PSA and comments were pro- or anti-vaccine; they were excluded from the analysis, leaving 205 participants.

For both the PSAs and the comments, people’s attitudes and intentions depended on how credible they thought the sources were. Health authorities do have some weight, as people who thought the PSA was credible were more likely to have their opinions swayed by it, whereas people who thought the commenters were credible weren’t as likely to base their opinions on the PSAs.

On the Internet, nobody knows you’re a dog

In this study, the online commenters didn’t have any identifying information that could affect their credibility. In a second study, the comments were presented with additional details: they were identified as coming either from a doctor of infectious diseases and vaccinology, a lobbyist specializing in healthcare issues, or an undergraduate student in English literature. A pretest surveying 101 participants on Mechanical Turk confirmed that the doctor was seen to be more credible than both the other commenters; there was no significant difference between the lobbyist and the student (sorry, English lit majors).

As in the previous study, Mechanical Turk participants (272 after filtering out those with comprehension or recall issues) saw either a pro- or anti-vax PSA, followed by pro- or anti-vax comments from one of the three commenter personalities. Participants were then assessed on their attitudes and intentions toward vaccination and their assessment of the sources’ credibility.

The results showed that when the online commenter was perceived as being credible, his or her stance actually had more sway over people’s opinions than a credible PSA. When the commenters and PSA agreed in message, the message was reinforced through credible commenters, becoming more persuasive.

As with all individual studies, this research isn't definitive by itself. Survey-based information can be unreliable, and the researchers didn’t take into account participants’ starting stances on vaccination. As the researchers themselves point out, this study also didn’t examine the effect of mixed comment threads on the persuasiveness of the message.

That said, these results are disconcerting. Anyone can claim credentials online, and even those with genuine credentials can be corrupted or misguided. If people place more trust in “credible” online commenters rather than public health PSAs, it pinpoints a devastating weak spot of public health campaigns.

On the other hand, the authors suggest that this information can be used to make PSAs more effective. Public health organizations can encourage positive comments from experts in the field and can also moderate their comment threads carefully. Campaigners can also try to ensure that their own messages aren’t perceived to be manipulative or biased.

Journal of Advertising, 2015. DOI not available.