Powerful measles protection rests on numbers: 92 to 94 percent. That is the portion of people in a community that must be vaccinated to prevent outbreaks of a disease that killed more children around the world in 2013 than car accidents or AIDS. At that immunization level, the virus has trouble finding victims, even the unvaccinated, as shown by both direct experience and models of disease. Libya and Tanzania, for instance, have passed that bar, with vaccination rates of 99 percent. The U.S., dragged down by parents who refuse to vaccinate their kids, has not. Coverage is as low as 86 percent in states such as Colorado and Ohio, and the national average is 91 percent. Last year, as a consequence, 644 people in the U.S. were sickened by measles—more than in any year since 2000. What is to be done to keep things from getting even worse?

Some policy makers have called for new laws that would require MMR (measles, mumps and rubella) shots for all school-age children. Eight out of 10 Americans would support such laws, one CNN poll found. But strong-arm tactics would probably backfire and reinforce the antivaccine movement, which is driven by fears that shots cause autism and other side effects. A better way, highlighted by behavioral science research, is to change other parents' hesitant attitudes with little nudges.

Parents who vehemently oppose the shots become even less likely to vaccinate when confronted by insistence on vaccine safety, researchers have found. Fortunately, such opponents make up only about 9 percent of adults, according to a recent Gallup poll. Their fear of a vaccine-autism link usually stems from a fraudulent and retracted 1998 Lancet study. But experts say there are many more people who do not vaccinate their kids because they are just unsure. For example, 52 percent of adults in the Gallup poll have not made up their mind about vaccines and autism. These fence-sitters could be prodded toward getting shots.

The best route to reach these parents is through their pediatricians, whom surveys show to be one of the most important sources of information about vaccines. When doctors do not stand strongly behind vaccines, refusal rates go up. A 2013 study in Pediatrics, for example, found that presenting vaccines as a vague choice (saying, “What do you want to do about shots?”) rather than the standard of care (“Well, we have to do some shots”) increased the odds of parents refusing almost 18-fold. And informing parents that their child would receive a vaccine at their next scheduled appointment made them more likely to go through with it than if they were merely asked which vaccines they wanted their child to receive.

For parents whose distrust of the medical establishment fuels hesitancy about vaccines (roughly 23 percent of the reluctant group), recruiting friends and peers to pass on the message could be more effective. Peer pressure and social norms are one of the strongest influences on the decision to vaccinate or not, scientists reported in 2014 in the Proceedings of the Royal Society B. To take advantage of this fact, in Washington State, a public-private pilot program trains provaccine parents to be advocates in their communities. These parents spread the message on social media and speak directly to their neighbors at schools and community events.

Simply asking people to plan ahead is another way to get more of those who are waffling to vaccinate. In a 2011 study, for example, employees at a midwestern utilities firm were sent a letter reminding them that they were eligible for a free flu vaccine. If the letter prompted them to write down both a date and a time for the shot, they were 4.2 percent more likely to get vaccinated than other employees were. (Flu shots are not as controversial as MMR shots, but the important factor is the change in compliance.)

If states begin implementing such interventions and search for new strategies, we can push past the safety threshold. Otherwise, we will fail to prevent a very preventable disease.