Millions of people took the first COVID-19 vaccine in the months following its release in December 2020. Since then, however, uptake of the vaccine and subsequent boosters has slowed considerably.

Are barriers to vaccine access part of the problem? A massive study involving more than 3.6 million people – all of whom had already received their primary series – aimed to find out whether offering free Lyft rides would encourage more people to get boosted, which can help stave off serious illness and death.

Results showed that text messages offering a free round-trip ride to a vaccination site was no more enticing to participants than seven other simpler, text-based reminder interventions designed using behavioral science insights. This doesn’t rule out the idea, however, that free rides might be enticing in other situations, the researchers said.

“A key distinction here – the participants in our studies had already figured out how to get to a vaccination site for their first round of vaccines, and so maybe the free ride doesn’t really add much in this context,” said Neil Lewis Jr. ’13, a Nancy ’62 and Peter ’61 Meinig Family Investigator in the Life Sciences, and associate professor of communication, medicine and public policy.

“That’s a really critical distinction here,” he said. “We didn’t find free rides mattering in this study, but I don’t think that rules out the possibility that free rides matter in other contexts.”

Lewis is a co-author of “Megastudy Shows Reminders Boost Vaccination but Adding Free Rides Doesn’t,” which published June 26 in Nature. The lead author is Katherine Milkman, professor of operations, information and decisions at the Wharton School of the University of Pennsylvania.

The other 17 co-authors – a multidisciplinary group including medical doctors, economists, statisticians and data scientists – are from Penn, the University of Maryland, the University of Chicago, Ascension Health and CVS Health.

According to Milkman, the result “was not what we’d expected, or what expert forecasters predicted. There is extensive research suggesting it’s critically important to reduce small frictions that can hinder vaccination. But our study offers an important lesson: Sometimes what we expect to work won’t, and science can tease out the truth from misconceptions.

“In this case,” she said, “well-designed reminders turned out to create just as much value as a far costlier approach to encouraging vaccination.”

Milkman is co-founder and co-director of the Behavior Change for Good Initiative (BCFG), an international group of researchers (including Lewis) who focus on “megastudies” – large-scale field studies with organizational partners, aimed at solving big problems. BCFG coordinated this interdisciplinary project.

“You really need that full village of different expertise to do this kind of thing,” said Lewis, co-director of the Cornell-based Action Research Collaborative. “Big problems require all of these people coming together to work on them.”

Because many people received their first COVID vaccinations at CVS Pharmacy or had shared proof of prior vaccination with their pharmacy and were signed up to receive text messages from CVS, the research team was able to partner with CVS to test whether different messages would motivate people to get their boosters.

This research included nine study conditions – eight text-based interventions related to getting the vaccine, plus a control condition with no messages. Text messages were sent on Nov. 3, 5 and 8, 2022, with reminders sent a week later. All messages conveyed the idea that the vaccine was “recommended” and “waiting for you,” themes found to be effective in previous BCFG vaccination studies.

In one key intervention condition, this language was paired with an offer of a free Lyft ride to and from a CVS Pharmacy. The authors found that the free ride was no more of an enticement than the other reminder messaging conditions.

On average, the eight interventions increased booster vaccination uptake rates by nearly 21%; for the control group, just over 5% of the pharmacy’s patients received a booster within 30 days of the study’s launch. The group estimated that nationwide, their interventions resulted in an additional 33,864 COVID boosters – plus 10,756 flu vaccinations, a spillover effect – during the experimental period.

In addition to the limited effect of a free ride, Lewis said he was disheartened by the general low uptake of the bivalent booster, which had been received by just over 10% of eligible Americans at the time of the experiment. Vaccine hesitancy could be part of the reason for people’s unwillingness to get boosted, Lewis said.

“That’s one of the things that we were getting at in testing these many different kinds of messages,” said Lewis, one of nine behavioral science experts who developed the eight intervention messages. “You might be hesitant because you’re worried about side effects, and the talk is that you might be knocked out for a couple of days, and you can’t afford to take off work and lose two days’ wages.

“And then there’s the narrative that people who were vaccinated might still be getting COVID anyway,” he said, “and that’s another reason people might say, ‘Is this even worth it? Am I really protected?’ Because of all these possibilities, we wanted to see which kinds of messages might help alleviate some of those concerns and convince someone to get vaccinated anyway.”

Support for this work came from Flu Lab; the Social Science Research Council’s Mercury Project, which receives funding from the Rockefeller Foundation, the Robert Wood Johnson Foundation, Craig Newmark Philanthropies and the Alfred P. Sloan Foundation; and the Wharton Behavioral Lab.