When it comes to persuading parents in the United States who are hesitant about vaccinating their children, the public health messages often rely on facts and science to explain how immunisation not only protects those children but also shields other vulnerable people from dangerous infectious diseases.

But information campaigns that emphasise fairness or preventing harm sometimes backfire and can worsen vaccine hesitancy, research has shown.

A study published Monday in Nature Human Behaviour suggests a more effective way to reach vaccine-hesitant parents may be to focus on two potentially powerful moral values that underlie people's attitudes and judgments: individual liberty and purity.

Compared with parents who approve of vaccines, parents who are most reluctant to vaccinate are strongly concerned with liberty and purity, the researchers found.

In this framework, liberty is associated with belief in personal responsibility, freedom, property rights and resistance to state involvement in citizens' lives. Concerns about purity centre on boundaries and protection from contamination.

Although the vast majority of parents across the country vaccinate their children and follow recommended schedules for this basic preventive-medicine practice, vaccine skepticism and outright refusal in recent years have led to places where there are communities of undervaccinated children who are more susceptible to disease and pose health risks to the broader public.

Minnesota's measles outbreak in the spring sickened 79 people and exposed 8,000 to the virus, including more than 500 people who were asked to stay home from school, child care or work because they were unvaccinated and at risk of contracting and spreading the disease.

The new study used a social psychology theory known as Moral Foundations Theory to assess the underlying moral values most strongly associated with vaccine-hesitant parents.

Their findings correspond with the reasons many vaccine-hesitant parents give for delaying or refusing some vaccines, and with many of the claims on anti-vaccination websites.

A main message of one of the most organised and politically active anti-vaccine groups, Texans for Vaccine Choice, for example, focuses on parents' rights to choose and individual liberty.

"Parents with the most concerns about vaccinating their children were twice as likely to have a high score for those clean, pure, wholesome themes as well as support of themes of individual liberty, compared to parents with the fewest concerns," said Avnika Amin, a doctoral student in epidemiology at Emory University, and one of the study's authors.

Much research about vaccine attitudes and interventions to boost vaccination rates focuses on education, she said.

"The assumption is that if we throw some fact out there, it will change their minds," she said. But that doesn't really address the root of the problem, she said, because people don't make decisions solely based on facts.

Amin and Emory colleagues conducted online surveys of 1,100 US parents of children under age 13. They assessed the parents' level of vaccination hesitancy and explored how important different moral values were to them when deciding if something was right or wrong.

They found that other factors, such as age, sex, level of education or political views didn't seem to affect the relationship between concern about vaccinations and their concerns for liberty and purity, she said.

Another group of researchers at Loyola University in Chicago, working independently from Amin's group, validated the main findings of the Emory group in another study.

The Loyola researchers went one step further and also found that purity and liberty values seem to influence belief in false or misleading statements on anti-vaccination websites.

Other recent research suggests that religiosity, as well as concerns about moral purity, consistently predicted vaccine skepticism.

In a study published Friday in the Personality and Social Psychology Bulletin, researchers at the University of Amsterdam and University of Kent explored the ideological roots of science skepticism about climate change, vaccination and genetic modification in food.

Based on online surveys of people in the United States, the researchers found that a person's strong religious beliefs were the best predictor of vaccine skepticism, and religious conservatives had low support for science overall.

Religiosity, they said, was the best predictor, "over and beyond political ideology, moral concerns, and scientific literacy."

In both papers, the authors said the results show a correlation but not a causal link.

Amin said incorporating the moral foundations of a person's health decision could be one way to improve public health communication.

If a mother has concerns about the chemicals in vaccines, her physician might address her concerns with that "purity" value in mind, she said.

For example, one intervention might urge parents to vaccinate as a way of boosting a child's natural defenses against disease and keeping the child "pure of infections," and show a picture of a child with measles, the researchers said.

Another liberty-oriented message might urge parents to take personal control of a child's health because vaccinations can help the child and others be free to live a happy and healthy life, they said.

Such messages could be displayed at schools, doctors' offices and on the Internet, where parents frequently get their information about vaccines.

The researchers cautioned that their suggestions have not been formally tested.

"But maybe it can help us, at least, better craft a message that is framed in a way that underscores these values that concern parents when they're making these vaccination decisions," Amin said.

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