As communities deal with the United States' worst measles outbreaks in 25 years and the country faces losing its "measles-free" status, Americans are debating how best to encourage the reluctant to get vaccinated. Measles is an infectious disease that can be deadly and has no cure (although some treatments can make the symptoms less severe). So, both during outbreaks and during less critical times, the best bet for preventing the illness from spreading is to get people immunized. The worst measles outbreaks tend to happen in communities with low vaccination rates, often because many in the community have incorrect beliefs about the measles, mumps, and rubella, or MMR, vaccine.
It's clearly possible to improve vaccination rates. But the science of what works—to assuage unfounded fears and increase how many people get shots—is still in its early days. In the U.S. and abroad, health officials have tried tons of different strategies. Sometimes they've even studied whether they were effective. In many cases, however, it's hard to identify the principles for designing an effective get-vaxxed program, particularly for people who don't vaccinate not because they're busy or poor, but because they oppose vaccination.
"There's a lot of things that are out there that have been proposed. It's a bit scattershot, in terms of the state of the science," says Richard Carpiano, a sociologist who studies health behaviors at the University of California–Riverside. He notes that the field is new because the current spread of anti-vaccination beliefs in cultures around the world is new. "We thought about vaccines as sort of settled science. People would do it," he says. "It really wasn't a problem 20 years ago."*
The better evidence is for programs that don't touch people's beliefs around vaccinations at all. "Strategies that are most effective are based not on trying to persuade people, but instead trying to nudge them along," says Noel Brewer, a psychologist who studies health behaviors at the University of North Carolina. In a review, a team of psychologists, including Brewer, found that aids, such as sending people reminders that they or their kids are due for shots, increase the number of people who get immunized against various diseases. It's important to remember there are plenty of people for whom nudges will work, Brewer says. After all, more than 90 percent of American children have received their recommended MMR shots—a rate that those working in other public-health areas could only wish for. "Vaccination is one of the most widely embraced health behaviors we have," Brewer says. "There's no other health behavior we can point to that's like this."
The problem is that those few people who are hostile to vaccinations tend to be grouped in the same communities, which makes them vulnerable to disease outbreaks. In neighborhoods with lower vaccination rates, what's called "herd immunity" breaks down. When enough people are vaccinated, illnesses can't easily pass between people, which helps protect those who can't be vaccinated because they're too young, or have certain medical conditions. Measles is highly contagious and may require 96 to 99 percent of people to be vaccinated for herd immunity to work.
In places with far lower rates, can people change their attitudes? More importantly, will they change their behaviors? In 2017, the European Centre for Disease Prevention and Control published a catalogue of strategies organizations have tried to overcome vaccine hesitancy. Several education programs did make people feel more confident about vaccines and reduced their belief in misconceptions. But few programs were studied for whether they increased vaccination rates.
There's even debate about whether mandating immunizations works. Among European countries, there's little correlation between whether a nation has strict mandates and its immunization rates, a recent analysis found. On the one hand, vaccine requirements send the message that these shots are important and widely accepted in society, explains Heidi Larson, director of the Vaccine Confidence Project at the London School of Hygiene and Tropical Medicine. On the other hand, they might trigger backlash in some cases.
Among U.S. states, the relationship is clearer. In states that allow parents to more easily opt their children out of school-required immunizations, fewer kindergarteners have their recommended MMR shots. Last month, New York City began requiring residents of one neighborhood hard hit by measles to get MMR shots, or face fines. The mandate came after officials had already tried working with rabbis in the majority Orthodox Jewish community, passing out fliers, and banning unvaccinated kids from going to school, the New York Times reports. All of the experts I contacted supported the mandate as a necessary measure to control one of America's largest and longest-lasting current measles outbreaks.
What does all this mean for those working on improving vaccination rates? They're up for trying everything, at least if my conversation with Carpiano is any indication. He listed a range of ideas: the best approaches for pediatricians talking to parents, working with technology companies to ensure misinformation about vaccines doesn't show up as top results in searches, and empowering teens to get vaccinations on their own, if they want to but their parents object. So the bad news is that there's no guaranteed way to improve vaccination rates, but the good news is that there are many ideas worth trying.
*Update—May 3rd, 2019: A previous version of this article incorrectly identified Richard Carpiano as a professor at University of California–Irvine.