A masked Buddhist monk uses a smartphone to register for a coronavirus vaccination appointment in India.

Even a brief mobile-phone reminder that COVID-19 vaccinations are available can increase vaccination rates.Credit: Sumit Dayal/Bloomberg via Getty

A short text message reminding people to book a COVID-19 jab can boost vaccination rates by several percentage points, according to a study of more than 90,000 people in California.

A reminder sent one day after individuals became eligible for the jab boosted appointments and vaccination rates by 6 percentage points and roughly 3.6 percentage points, respectively, compared with rates in a separate group that did not receive the reminder, researchers report on 2 August in Nature1. A second reminder to those who still hadn’t booked a vaccination appointment a week later boosted appointments and jabs by another 1.7 and 1.1 percentage points, respectively.

These small gains, if applied to a large population, could speed up vaccinations for millions of people, the authors say. “The most surprising thing is how powerful such a low-cost intervention can be, and I don’t think we’re using it enough,” says Daniel Croymans, a physician at UCLA Health, the health system affiliated with the University of California, Los Angeles, and a co-author of the study.

Hesitancy takes hold

Even in some countries where vaccines are plentiful, public hesitancy has stalled their uptake. And a partially immunized population is a fertile breeding ground for new viral variants — some of which might eventually evade existing vaccines.

Such concerns have raised scientists’ interest in ‘nudges’, which use positive reinforcement and indirect suggestions to change human behaviour. Text-message nudges boosted influenza-vaccination rates by 2.1 percentage points according to a megastudy2 published in May, and Croymans and his colleagues hoped to find out whether the same would be true for COVID-19 jabs.

The researchers tested reminder texts with two different wordings, which were sent to members of a large health-care system. Some people received a ‘basic’ reminder telling them that they could get the vaccine. Previous research showed that a sense of ownership amplifies the desire to act3, so other people received an ‘ownership’ reminder telling them that a vaccine had “just been made available” to them, with instructions on how to “claim” their dose.

The ownership reminder was more effective: when sent one day after people became eligible for the vaccine, it raised the recipients’ vaccination rate by about 4.1 percentage points, compared with a 3-percentage-point increase for the ‘basic’ reminder.

“I’m still sometimes amazed at how subtle changes and wording can make a difference,” says David Asch, a behavioural economist at the University of Pennsylvania in Philadelphia. He adds that this study is a “glass half full, glass half empty” story: although the nudges made a notable difference, they won’t solve the problem of entrenched vaccine hesitance, given the modest effect sizes, he says.

Convincing the holdouts

Despite the nudges’ success, Croymans concedes that if he were to run the study today, he might see significantly different results. That’s because the group of people who are still unvaccinated now might include a bigger proportion of those who are very hesitant than did the group who were unvaccinated when the study was conducted, six months ago. At that time, vaccines were first becoming widely available in the United States.

To address more entrenched holdouts, Croymans and his colleagues are thinking about other nudges. For example, health messages could be customized to take into account publicly available information on recipients’ affiliations to particular political parties.

“Especially at this stage, where there remains a group of people pretty hesitant to get the vaccines, tailoring the message to them is going to be even more important,” he says.