More than 169 million Americans have received one or both doses of a coronavirus vaccine, but demand is falling off rapidly. Doses administered peaked at an average of 3.3 million per day in mid-April but are now down to under 1.5 million per day. To meet the goal of getting at least one shot to 70 percent of American adults by July 4, the Biden administration announced Wednesday that it would partner with child-care centers to free up people to get injections, and provide vaccine information (and even doses) through Black-owned barbershops and beauty salons. Anheuser-Busch will give a beer to Americans over 21 if the country meets Biden’s goal, and some states, notably West Virginia, Maryland and Ohio, are using cash incentives.

Our research demonstrates that a less-flashy effort might also have a significant effect in boosting vaccination rates: text messages. In a new study involving more than 47,000 people, we identified messages that were able to “nudge” people who were scheduled for a primary-care doctor’s visit to get a flu shot during that visit. Although the study took place last fall, before coronavirus vaccines were widely available, we designed the messages so they could be repurposed for the new vaccines.

After soliciting suggestions for different approaches from dozens of experts, our large, interdisciplinary team tested 19 text messages that ranged from jokey and casual to sober and professional. One of the latter turned out to be the most effective: 72 hours before a doctor’s visit, we told patients that it was flu season and that “a flu vaccine is available for you.” Then, 24 hours before the appointment, we informed them that “a flu vaccine has been reserved for your appointment.” The proportion of people in the control group who got a flu shot was 42 percent. The “reserved for you” nudge raised that figure by 4.6 percentage points, as we reported in the Proceedings of the National Academy of Sciences last month. (All patients in the study — including the control group — were supposed to be offered a vaccine by their doctor, but we can’t be sure that happened in every case.)

Likewise, in a similar study we conducted involving roughly 700,000 Walmart pharmacy customers, texts reminding patients that a flu vaccine was “waiting for you” lifted vaccination rates more than any other line of messaging did.

Why do text message reminders using phrases such as “reserved for you” and “waiting for you” work so well? It might be because this language makes patients feel as if the vaccine belongs to them, and they may not want to miss out on “their” dose. Research also shows that people are more likely to accept default options than to choose to opt in to the same course of action. One reason is that defaults are presumed to be the recommended path. Accepting defaults is also perceived to take less effort. After all, you’re just going along with what has already been arranged and accepting a vaccine that’s ready and waiting. (That impression apparently matters, even if it takes little to no effort to simply say yes when a doctor offers you a vaccine after you receive a different text, or no text at all.)

Five other messages we tested to encourage vaccine adoption at the doctor’s office showed statistically significant benefits as well: One informed patients that a flu shot had been reserved for them and prompted them to “reply ‘Y’ if you want this shot held for you”; another showed patients a short video about what being sick with the flu is like.

In contrast, messages were less effective when they were incongruent with the sort of communications that patients are used to receiving from their health-care providers. The most flippant message, for example, was also one of the worst-performing: A dog tells a joke to a cat: “Did you hear the joke about the flu? Never mind, I don’t want to spread it around.” A message that framed vaccination as a competition also fared poorly: “In 2019, flu shots in Pennsylvania lagged 2% behind Maryland. Help Pennsylvania become a leader in flu prevention and get your flu shot.” (Still, none of the texts decreased rates of flu vaccination relative to the control group.)

In the Walmart study, although the challenge was different (drawing people into the pharmacy to request a shot rather than nudging them to accept a vaccine during a doctor’s appointment), we found similar results. Messages that came across as informal performed poorly in that context, too. Repeated reminders added value at Walmart (confirming that nagging can work), and the presumptive default language won out again.

The promising results from these two large experiments can be adapted to encourage coronavirus vaccinations. And early findings from a new study at the University of California at Los Angeles, run by several of our collaborators, suggest they’ll probably yield payoffs. Hengchen Dai and her colleagues tested the value of text messages sent to tens of thousands of UCLA Health patients one and eight days after they were notified of their eligibility to receive a coronavirus vaccine, and they found that text reminders increased inoculation rates by up to 3.4 percentage points. Again, the best-performing messages across four they tested reminded patients that a vaccine was “available for you” and encouraged patients to “claim your dose.”

Compared with other tactics, including glitzy ad campaigns and cash incentives, strategies based on text messaging are virtually costless. Yet they move the needle (literally). And unlike mandates, nudges preserve the autonomy of individuals to make their own decisions. We aren’t advocating against cash rewards or mandates — far from it. In our view, in the race to end the pandemic, the more approaches, the better.

But let’s not overlook an easy and cost-effective tactic based on solid behavioral science. And as we think about vaccine nudges and incentives, let’s remember that they can have a snowball effect. Momentum builds, as more and more Americans can say to one another: “Yes, I’ve been vaccinated.” After all, nothing changes a person’s mind like altering what the crowd around them is doing.

correction

In an earlier version of this article, the first author's name was misspelled in the byline. The article has been corrected. The article has also been updated to incorporate new vaccine incentives offered by the Biden administration.