Impact of reminder messages, with and without financial incentives, on influenza vaccination: A randomized trial in a California health system

Vaccine hesitancy has risen in the aftermath of the COVID-19 pandemic, raising new concerns about declining uptake of routine immunizations like the seasonal flu shot. Although past studies have shown that financial incentives can boost vaccination, it is unclear whether these strategies remain effective in today's more skeptical environment. To examine this, we conducted a three-arm randomized controlled trial in a racially and economically diverse county health system in Northern California. The trial, which was conducted between January 17 and February 16, 2024, tested the impact of financial incentives and reminder messages on flu vaccination uptake, focusing on patients who remained unvaccinated late in the flu season and despite multiple prior outreach attempts by the health system. A total of 69,972 adult patients overdue for influenza vaccinations were randomized to one of three arms: (1) standard care, (2) a reminder message, or (3) a reminder with a $50 financial incentive to get vaccinated within one week. We hypothesized that a reminder message with a financial incentive would increase vaccination uptake more than standard of care. We further hypothesized it would increase uptake more than a reminder message alone. We found that while reminder messages alone had no impact on vaccination uptake, the addition of a $50 incentive nearly doubled the 1-week vaccination rate-from 0.343% to 0.613% (a 0.27 percentage point increase; p < 0.001). Thirty days later the reminder message still had no impact relative to standard of care (0.14 percentage points, p-value 0.218) but the financial incentive effect persisted, increasing the vaccination rate from 1.55 to 1.92 percentage points (0.37 percentage points p-value 0.002). These findings suggest that financial incentives remain a powerful tool for increasing influenza vaccine uptake, even among those who remain unvaccinated late in the flu season. Trial Registration: NCT06300242.