Incentives, Surrogates, and Long-run Vaccination

Can monetary incentives improve health behaviors in the long run, and do commonly used surrogate outcomes capture these effects? We study these questions in the context of vaccination using a large-scale field experiment. The experiment combines commonly used surrogates—vaccination intentions, intermediate behavioral proxies, and short-run vaccination—with long-run administrative vaccination records. We first document that incentives increase vaccination rates in the long run: guaranteed $20 incentives raise COVID-19 booster uptake by 9 percentage points. Lottery-based incentives also increase long-run uptake, while prosocial incentives primarily accelerate vaccination. Second, using surrogacy methods, we study whether surrogates can predict long-run impacts. Although the surrogates are strongly correlated with eventual vaccination, the assumptions required for surrogacy methods are often violated, and they do not accurately predict long-run impacts. Our findings highlight both the ability of incentives to change behavior and the importance of measuring long-run outcomes rather than relying solely on surrogates.