Vaccine hesitancy was outlined as one of the top threats to global health alongside climate change, according to a statement by the World Health Organization (WHO) in 2019. Studies such as de Figueiredo et al. (2020) note that confidence in the importance, safety, and effectiveness of vaccines fell between 2015 and 2019 in many countries around the world, including developed countries such as South Korea and Poland. Somewhat surprisingly, European countries were considered among the least vaccine-confident, with France mentioned as the most skeptical (Karafillakis et al., 2022).

The issue of large-scale immunization programs and vaccine hesitancy gained unexpected prominence soon after WHO’s statement. The outbreak of COVID-19 in early 2020 accelerated vaccine development, leading to the conditional approval and authorization of the first vaccines within months. Despite being fully subsidized and offered free of charge to all European Union (EU) citizens, vaccine uptake remained suboptimal in several countries due to concerns over safety and effectiveness.

This was also the case in Slovakia. According to a nationwide representative poll conducted in December 2020, only a third of the population indicated a willingness to get vaccinated against COVID-19. As of December 2021, only 49% of the population was vaccinated with at least one dose. Over the course of the pandemic, reluctance to immunization was likely one of the most significant drivers of severe overcrowding in hospitals, with cases requiring intensive care. The health care system in the country reached its capacity limit several times, with the number of concurrent hospital admissions peaking at 3600.

To address low vaccination rates among high-risk groups, the Slovak government introduced a conditional cash transfer (CCT) scheme, Vakcinačný bonus,1 offering €200 or €300 to individuals aged 60 or older as of December 31, 2021, conditional on receiving a COVID-19 vaccination. Nearly 847,000 individuals became eligible for the transfer, with total costs amounting to €245 million.

Conditional cash transfers have long been used to reduce poverty, increase human capital, and incentivize targeted behavior. According to the human capital theory (Grossman, 2000), individuals will invest in health if the expected private benefit exceeds the cost. Thus, a decision not to take a vaccine as a preventive measure can be interpreted as disutility from vaccination (including fear of negative side effects), discounting due to time preferences or expected mortality, or a low valuation of life. The presence of externalities means that individuals weighing only their private costs and benefits of vaccination, while ignoring the broader social benefits, contribute to socially suboptimal vaccination rates.

There is also a potential moral hazard problem: While it may be socially efficient to vaccinate, individuals may neglect this preventive measure because they do not incur the full costs of treatment if they fall ill due to insurance. A simple theoretical framework by Kremer and Glennerster (2011) suggests that insurance providers (or the government) may wish to subsidize vaccination or even mandate it. The latter, as experience shows, is often legally problematic. Therefore, a more feasible solution may be to offset the disutility from vaccination by means of a CCT.

There is substantial literature showing the effectiveness of CCT programs on the uptake of immunization, mostly focused on randomized controlled trial (RCT) evaluation. For example, Banerjee et al. (2010) study immunization coverage of children in 134 Indian villages using a clustered RCT, evaluating immunization campaigns with and without financial incentives. Their findings suggest that modest incentives, such as one kilogram of dried beans, have a positive effect on immunization in settings with very low immunization rates.

Salinas-Rodríguez and Manrique-Espinoza (2013) analyze the effects of the Mexican CCT program Oportunidades on vaccination coverage in older Mexican people. Under the Oportunidades scheme, households were eligible for a cash transfer of roughly $25, conditional on the attendance of each household member older than 70 years at preventive health check-ups. Using cross-sectional survey data on a matched sample of 4628 individuals, the authors report that the Oportunidades program significantly increased vaccinations for a complete schedule by 5.5%, for influenza by 6.9%, for pneumococcal infections by 7.2%, and for tetanus by 6.6%. The authors hypothesize that the effect on vaccination coverage could reside in the preventive check-ups that the program made mandatory for eligibility.

Kusuma et al. (2017) evaluate a large-scale RCT with CCTs on child vaccination rates in Indonesia, with a sample of over 4000 children under two years old. The cash transfers ranged from $60–$220 per household per year, depending on the number and age of children. After two years of implementation, their difference-in-differences estimates show that the program significantly increased child vaccination rates for all basic vaccine types by up to 30%.

Few studies examine COVID-19 vaccine uptake in the context of CCT programs. Campos-Mercade et al. (2021) evaluate an RCT in Sweden with 8286 participants, where the effect of a small cash reward (around US $24) is compared with the effect of several behavioral nudges. Their results suggest that the monetary incentive has the power to increase participation by about 4 percentage points, while behavioral nudges and reminders also have a small positive effect. Wong et al. (2022) exploit a quasi-experimental incentive program that offered a $25 cash card to adults who received their first dose of the COVID-19 vaccine in select clinics within four counties in North Carolina. The authors analyze the effects of the program using a difference-in-differences approach within a competing-risk hazard rate framework. Clinics participating in the intervention reported an increase in vaccination rates by 46.2%, while remaining clinics in the counties experienced a 9.5% decline. As part of a survey, 41% of vaccine recipients indicated that the cash card was an important reason for vaccination. Jacobson et al. (2022) evaluate outcomes of an RCT on a sample of 2701 Medicaid patients in California, where participants were assigned either to behavioral nudges or $10 or $50 financial incentives for vaccination. The authors conclude that none of the interventions increased vaccination rates.

Several US states implemented conditional cash lotteries (CCL) during the pandemic. Barber and West (2022) study the effects of Ohio’s Vax-A-Million CCL initiative, the first CCL targeting COVID-19 vaccinations. Forming a synthetic control from other states, their findings reveal that CCL increased the vaccinated share of the state population by 1.5% (0.7 percentage points), costing $68 per person persuaded to vaccinate. Furthermore, the authors also claim that the lottery prevented at least one infection for every six vaccinations that the lottery had successfully encouraged.

This study makes three key contributions to the existing literature. First, it evaluates a nationwide CCT program implemented in a developed country, affecting over 840,000 individuals and representing nearly one-sixth of Slovakia’s total population. Most research on financial incentives for immunization has focused on childhood vaccinations in low-income settings, whereas evidence from adult populations in developed economies remains scarce. Second, the program’s financial incentives were substantially larger than those examined in previous studies, where only modest cash amounts were typically offered. In Slovakia, the CCTs represented approximately 25% of the average monthly wage and nearly 60% of the average retirement pension. Finally, the study leverages administrative data and a regression discontinuity (RD) design to provide credible causal estimates of the program’s effects on vaccination uptake, health care utilization, and mortality. In addition, information about personal characteristics at an individual level, such as gender, income, or prevalence of chronic conditions, allows us to characterize the complier population.

Our findings suggest that the CCTs significantly increase the uptake of COVID-19 vaccines among the targeted population for all three vaccine doses, most notably for the third dose. Using administrative data on overall health care utilization and mortality, we also estimate the causal effects of the CCT program on these health outcomes at the eligibility threshold, finding some evidence of lower utilization among compliers. By profiling a potentially vaccine-hesitant complier population for the first dose, our results suggest that men are significantly more likely to accept the cash transfer for the first dose, as well as those with higher incomes. This contrasts with compliers for the booster dose, where always-takers are characterized by significantly higher incomes compared to compliers. There is also some evidence that the cash transfer promotes habit formation, as almost 37% of those nudged to get the first dose also accept the second dose.

Despite the increased vaccination rates, the estimated health benefits at age 60 are well below the overall costs. In extrapolating the benefits for older ages, our analysis rests on the assumption that the vaccination program would not have a significant impact on virus transmission rates. The program exceeds the cost-effectiveness threshold only if we assume a constant effect of the program for everyone aged 60 or older and only for nudging unvaccinated individuals to take the first dose. Under a more realistic scenario where the effect diminishes with age, none of the interventions are cost-effective. We also explore potential correlates of the estimated treatment effect, which varies substantially across districts. Higher median income, life expectancy, prior vaccination rates, as well as a higher share of the Roma, Hungarian, and female populations, are all associated with lower treatment effects.

The rest of the paper is organized as follows: Section 2 describes the institutional setting and the data. Section 3 outlines the econometric methodology. Section 4 presents the main results, while Section 5 presents the results from the benefit–cost analysis. Section 6 explores the heterogeneity of the estimated treatment effect. Section 7 provides a discussion. Section 8 concludes with final remarks.