Effects of Lottery Incentives for Influenza Vaccination: Evidence from a Large-Scale Randomized Trial and Causal Forest Analysis

Financial incentives may encourage vaccination, but their cost-effectiveness depends on targeting patients who are most responsive. We conducted a randomized controlled trial with 57,579 adult patients in an integrated health system during the 2021–2022 influenza season. Patients with an upcoming appointment were randomized to a no-contact control, a simple reminder, a $1 cash incentive, or a scratch-off lottery ticket incentive worth up to $5,000. Any active intervention increased vaccination at the scheduled appointment by 2 percentage points (an 8% increase over the control mean) and by 1.5 percentage points over the full influenza season. The three active arms did not differ significantly, indicating that small financial incentives provided no additional benefit beyond a reminder. Using electronic health records and area-level characteristics, we used causal forests to estimate conditional average treatment effects. Predicted responsiveness varied substantially: patients in the top quartile were 3–4 times more likely to respond to incentives than those in the bottom quartile, whereas reminder effects were relatively homogeneous. These findings suggest that low-cost reminders can increase influenza vaccination, while machine-learning methods can help identify patients most responsive to small financial incentives.