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Pharmacist Danny Tam with London Drugs prepares to give a flu shoot in Vancouver January 6, 3014.John Lehmann/The Globe and Mail

When it comes to convincing wary parents that vaccinations are safe, public health officials have their work cut out for them.

A study published Monday in the journal Pediatrics found that pro-vaccination messages may be ineffective – or could even backfire – among parents who report reluctance to have their children vaccinated. "It's very hard to convince people using evidence when they have strong views to the contrary," said lead author Brendan Nyhan, an assistant professor in the department of government at Dartmouth College in Hanover, N.H.

Nyhan and co-authors from the United States and the United Kingdom set out to test the effectiveness of current public-health messages designed to reduce misconceptions about vaccines and increase immunization rates for measles-mumps-rubella (MMR). They recruited a nationally representative sample of 1,756 American parents, and randomly assigned them to receive one of four different messages about vaccinations, or no message.

The pro-vaccination messages ranged from fact-based information to emotive accounts of the potential consequences for children unprotected against MMR.

One message provided evidence from the Centers for Disease Control and Prevention showing that the MMR vaccine does not cause autism. Another gave information about the dangers of measles and other diseases prevented by the vaccine. The third message showed pictures of sick children who have diseases that may be prevented by the vaccine. The fourth consisted of a dramatic narrative about an infant who almost died of measles.

In the study, however, not one of the messages appeared to sway parents who had unfavourable attitudes about vaccines. In fact, for parents most distrustful of vaccination, images of sick children with MMR diseases actually increased their reported belief that the MMR vaccine causes autism. Similarly, the narrative about an infant in mortal danger from measles increased their reported beliefs that vaccines cause serious side effects.

The findings suggest that logic does not play a major role in how people respond to public-health information about vaccines. Vaccines are a controversial topic, Nyhan pointed out, so "it's not surprising that people respond to information about them in a way that reflects their pre-existing views."

In light of recent measles outbreaks in North America and Europe, however, misconceptions about the MMR vaccine are proving to be a public health hazard.

Canada's immunization rate has been declining for a decade, dropping to 84 per cent – below the threshold to ensure population immunity. A recent report card by Unicef found that Canada's immunization rate is lower than that of countries such as Tunisia and Eritrea.

Fears about vaccination have persisted ever since the late-1990s, when British physician Andrew Wakefield warned that the MMR vaccine causes autism. The medical community thoroughly debunked his theory, and Wakefield was stripped of his medical licence. Nevertheless, the return of measles in pockets of North America and Western Europe may be largely due to the panic Wakefield instilled in parents, whose children were never vaccinated.

Yet current public health messages designed to correct misinformation about vaccines may be counterproductive, according to the Pediatrics study. After reading correct information provided by the study, anti-vax parents said they were less convinced that the MMR vaccine causes autism – but these parents also reported reduced intentions to vaccinate their children following the message.

Nyhan theorized that, after reading the correction, these parents may have focused on other concerns about vaccines. In some cases, when confronted with the facts, "people double down on the beliefs that are challenged," Nyhan explained.

Nyhan and co-authors cited evidence showing that parents rate their children's doctors as the most trusted source of vaccine safety information. Nyhan acknowledged that high levels of trust in family doctors may not apply to the most hardcore anti-vaccination parents, but he identified a broader group of parents who have concerns about vaccines but are not entirely convinced vaccines are unsafe.

"Doctors could be persuasive to those people on the margins," Nyhan said.

He added that the study results don't imply that public health authorities should stop disseminating pro-vaccine messages. "It just means that we should try to test those messages first and learn which are more effective," he said.

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