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Last year, a consortium of public health groups and Facebook embarked on an experiment to test how well people respond to different sorts of vaccine messaging. Over a period of nine months, they showed vaccine content to more than 100 million people on Facebook across six countries, tweaking everything from the message itself to its tone, format and style, then analyzing how people responded.

When marketing products, companies often test colors, language, typeface and other variables to determine which ads are most likely to make a consumer click. Why not apply the same sort of A/B testing to vaccines?

It turned out that not only did certain content evoke more positive opinions about vaccines, but the messaging that did best varied based on the country where it was tested. In Ukraine, for example, an informative tone did a better job at improving perceptions of vaccination than emotional pleas. In some countries, cartoons worked best. In India, the top-performing message was a personal appeal from a doctor talking about why he vaccinated his own kids. In Kenya, the winner was a straightforward message accompanied by an infographic with a recommended vaccine schedule for kids.

"We need to test our vaccine messaging for efficacy and safety just as we test our vaccines for efficacy and safety," says Angus Thomson, a social scientist at United Nations Children's Fund who worked on the study in conjunction with Facebook, The Public Good Projects and the Yale Institute for Global Health.

In Kenya, researchers tested several tones of vaccine ads to see which performed best.

The disparate results highlight the need to tailor messages to specific audiences. To that end, the public health groups have created the Vaccination Demand Observatory, which will continue working with Facebook to get a better sense of what vaccine content performs best where. The work is critically important as the world grapples with the combined obstacles of the hyper-contagious delta variant, sluggish vaccine rollouts in some nations and plateauing uptake in others. In the U.S., where vaccines are widely available, about 25% of eligible adults haven't taken their shots.

The Vaccination Demand Observatory and Facebook are currently working to replicate that initial experiment with more scientific rigor, testing content in four countries. The materials that perform best will then be compared with standard messaging in randomized trials to measure outcomes like vaccine coverage rates.

The trouble is that often the facts aren't really at the root of vaccine concern—and such A/B testing can help pinpoint exactly what is and address it directly. Often the problem comes down to a lack of trust, whether in the science itself, the government or the health-care system. That's part of why the tone of a message is just as important as the content.

That's where the Vaccination Demand Observatory is trying to make a dent.

"We should be valuing and investing in public trust as much as we value and invest in vaccines," Thomson says. "Because without any public trust, there's no public immunity."—Kristen V. Brown

A Surprising Southern Success

Demographics alone would suggest Bradley County, Arkansas, should be struggling fiercely with local resistance against vaccines, just as many other counties are all across the southern U.S. Yet in July, it had inoculated people at more than twice the rate of several other Arkansas counties. In this bonus episode we head to Bradley County to find out what's going on. The answer provides a case study on how to combat pockets of vaccine skepticism. Get the episode here.

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