Americans get a stream of messages telling them to avoid vaccines, from Jenny McCarthy on Oprah to billboard animations shown in Times Square. The responsible solution—fight back with forceful pro-vaccine messaging, right?

Actually, fighting fire with fire may backfire, according to a study in the journal Health Psychology.

Researchers asked over 100 participants to imagine parenting an 8-month old as they read about a pretend illness. In the scenario, their doctor advised that the child be vaccinated against the disease—after which subjects were shown typical anti-vaccine warnings that described how vaccines compromise the infant’s immune system.

Researchers then showed the participants two reassuring statements that vaccines pose little risk. Half the participants read: “There is only sporadic evidence that repeated vaccinations overwhelm the immune system.” The other half read: “There is no evidence that repeated vaccinations overwhelm the immune system.”

Those who were told there was no evidence for risk reported greater concern about vaccination and less intention to vaccinate their child than those who read the moderate messaging. The effect intensified when the messaging came from a perceived untrustworthy source, like a pharma company. So a softer sell may make a harder impression.

—Christie Nicholson