Because vaccination introduces some small degree of disease into someone’s system, it’s not safe for anyone who doesn’t have a healthy immune system. For that reason, people undergoing medical treatments that suppress the immune system (like chemotherapy) can’t be vaccinated nor can babies below a certain age.

But vaccines can still protect these people from infectious disease. If enough people around them are immune, the "herd immunity" prevents diseases from moving through the population. An experiment published in Nature Human Behaviour this week explores whether people who get information on herd immunity are more likely to decide to get vaccinated. The results suggest that it could be a worthwhile strategy for promoting vaccines, but it will need confirmation in real-world settings.

Free-riding vs. prosocial vaccination

Is emphasizing herd immunity in public health communications a good idea? It’s not clear. It might boost vaccine uptake rates, but it could very easily backfire. If people are made aware of herd immunity, they might start thinking that they personally are protected whether or not they actually get the vaccine. In the real world, there are also many other complicating factors in whether or not people choose to get their kids—or themselves—vaccinated. People have a lot of preconceived ideas about many vaccines and the diseases they protect against, and it can be hard to change their minds.

It seemed like a good idea to start exploring the issue of herd immunity in simpler scenarios with fewer of these real-world complications. To do this, a team of researchers led by Cornelia Betsch at the University of Erfurt, Germany, gathered 1,962 participants for an online experiment. Half were from Western countries (the US, Germany, and the Netherlands) and half from Eastern countries (South Korea, Vietnam, and Hong Kong). These groupings were created based on important cultural differences—where Western cultures tend to be more individualistic, Eastern cultures are more collectivist. (If this sounds like gross stereotyping to you, it is certainly a simplistic description of a complex cultural phenomenon, but there is evidence backing up its importance to behavioral research.)

The participants’ task was to choose whether to get the vaccine for a hypothetical disease. Betsch and her team tweaked various aspects of the experiment to test for different things. First, some of the participants were shown information on herd immunity—some read a text, and the lucky ones got to play around with an interactive simulation of how herd immunity works. A control group saw no information at all.

The information about herd immunity could be presented in one of two ways: either it emphasized that getting vaccinated helped to protect others (the social benefits), or it emphasized that herd immunity could help protect you if you were unvaccinated (individual benefits).

Then, they were given two different disease scenarios: one for a highly contagious disease and the other for a less-contagious one. Symptom-wise, both diseases sounded pretty nasty. After reading about the disease, the vaccine, and the side effects of the vaccine, the participants were told what percentage of people around them had been given the vaccine. That helped them to judge how likely they were to get infected: a highly contagious disease with a low vaccine rate was bad news.

The herd message works—in some contexts

For highly contagious diseases, people wanted the vaccinations no matter how high the population vaccination rate was. The people who had learned about herd immunity had slightly higher rates of vaccine uptake. These numbers overall weren’t very high, though—for Eastern populations, around 80 percent of participants said yes to the vaccine, and for Westerners, only 72 percent did.

With the less-contagious diseases, it really mattered to people whether the population around them had high rates of vaccination. If more of those around them had been vaccinated, people were more likely to say no to the vaccine, which suggests free-riding on herd immunity. But for Westerners, learning about herd immunity did increase uptake, from 53 to 59 percent. In Eastern countries, around 60 percent of people said yes to the vaccine, and learning about herd immunity didn’t make that much of a difference.

It turned out that emphasizing social or individual benefits of vaccination also made a difference. Pointing out the individual benefits worked better to convince people to take the vaccine if the overall population vaccination rates were low. If the population rates were higher, then it was better to emphasize the social benefits. And overall, the interactive simulation increased vaccine uptake more than the text-based one.

These results could help to steer public health campaigns, but there’s more research needed first. There are some important gaps between this and the real world: different cultures have very different levels of confidence in vaccines, and low disease levels in some countries muddy the water. There’s also the problem of combating misinformation on particular vaccines. As this research suggests, whether or not a vaccine seems like a good idea is likely to be highly dependent on the context.

Nature Human Behaviour, 2016. DOI: 10.1038/s41562-017-0056 (About DOIs).