Here we report findings from a randomized controlled trial (RCT) studying the impact of guaranteed monetary incentives on COVID-19 vaccination. We paid participants, drawn from a general sample of the Swedish population, SEK 200 (about $24) conditional on becoming vaccinated. The Swedish setting provides a unique opportunity to link individual-level survey data from the RCT to exhaustive population-wide Swedish administrative records for actual vaccinations collected by the public health authorities. We find that the monetary incentives increased vaccination rates by 4.2 percentage points. This is an increase from a 71.6% baseline rate, which is a similar rate to other countries in the EU, indicating that incentives can increase vaccine uptake even in countries with high vaccination rates.
We conducted the pre-registered RCT from May to July 2021 with 8,286 participants 18-49 years of age. Participants were recruited from a broadly representative online panel created by Norstat, a large survey company. We sent the survey to each participant as soon as the first Swedish regions opened vaccination for the participant’s age group. In the online survey, we randomized participants into five different treatment conditions and one control condition. Immediately after the treatment we measured participants’ intentions to get vaccinated against COVID-19. Except for the participants in one of the conditions (the no-reminders condition), all participants, even in the control group, received two reminders to vaccinate two and four weeks after taking the survey. In August 2021, the Swedish Public Health Agency linked the trial data of each participant to their COVID-19 vaccination records collected for all Swedish residents.
Our pre-registered main outcome variables are (i) participants’ self-reported intention to get a first dose of a COVID-19 vaccine within 30 days after vaccines become available to them and (ii) whether participants vaccinated within 30 days according to the administrative records. All reported results in the text and figures come from ordinary least squares regressions (OLS) with heteroscedasticity-robust standard errors (see SM section 1.2.2 for details; all p-values come from two-sided t tests).
In the incentives condition, participants were offered a monetary incentive of SEK 200 (about $24) if they vaccinated within 30 days of the vaccine becoming available to them. We checked uptake using the administrative vaccination records.
We collected detailed information on individual characteristics of the participants. We found large baseline differences in vaccination uptake across sociodemographic groups: People with a higher socio-economic status (college degree, higher income, employed) showed higher vaccination rates (see SM section 2.6). Strikingly, and despite the different baseline vaccination rates, we found that monetary incentives increased vaccination rates similarly across all subgroups (see SM section 2.5). This result indicates that monetary incentives have the potential to increase vaccination rates irrespective of people’s background.
Hence, we found larger impacts of monetary incentives on vaccination uptake than of behavioral nudges. While the pre-registered main analysis focused on the comparison between each of the experimental conditions and the control condition, we could also study the impact of the incentives condition relative to the three nudges. We found that the incentives condition had a larger impact on vaccination uptake than the three nudges pooled (3.1 percentage points difference, p = 0.038).
In sum, our study reveals that even modest monetary incentives can increase COVID-19 vaccination rates. We found that payments of SEK 200 (≈$24) increased COVID-19 vaccination rates by 4 percentage points. Our trial shows that incentives can increase vaccination uptake even when baseline vaccination rates are high. In contrast, behavioral nudges had small and not statistically significant effects on vaccination rates.
A natural question is whether paying people to get vaccinated is cost effective for governments. In addition to the direct benefits of saving lives, increasing vaccination rates leads to indirect benefits such as higher population immunity, lower hospitalization rates and medical costs, and economic growth. It is beyond the scope of this report to provide an encompassing cost-effectiveness analysis, but SM section 2.9 offers some perspectives on why our intervention likely is cost-effective. A key consideration is that paying for vaccination carries much lower costs for society than the sum of all payments; since money is transferred from the government to the citizens, the money paid is not lost.
Our study has several limitations. First, we only tested one size of monetary incentive. Companies and governments around the world have proposed incentives that range from less than $1 in Philadelphia and €25 in Serbia, to $100 in New York. Our trial cannot shed light on whether smaller or larger incentives would be more effective. We also cannot assess the effectiveness of other ways of incentivizing people, such as increasing health insurance premiums for the unvaccinated. Second, while during summer 2021 Sweden had a vaccination rate in line with the EU average, countries greatly differ in the proportion of vaccinated population, and the effect of incentives may vary depending on vaccination rates. Relatedly, we offered incentives when the rollout was starting; results could differ if monetary incentives are offered later, for example because unvaccinated people could increase their reluctance over time. Third, monetary incentives could potentially crowd out the willingness to get vaccinated in the future (e.g., booster shots) without getting paid. Finally, people might react differently based on who provides monetary incentives and corresponding trust in receiving the promised payments. In our case, researchers provided incentives, but the effects could differ if incentives were offered by governments or companies.
Despite these limitations, our pre-registered trial yields a clear result: guaranteed incentives can increase COVID-19 vaccinations. As the COVID-19 pandemic continues, incentives could be an effective tool to reduce COVID-19 spread and fatalities.