We will run a randomized controlled trial (RCT) to evaluate the effects of financial incentives on flu vaccination take-up. Adult Geisinger patients meeting the study eligibility criteria will be randomized to at least four arms; with sufficient sample, we may incorporate up to four additional arms for a maximum of eight. We will vary the availability, level, and structure of the financial incentives available to subjects in each arm. We hypothesize that, among incentives of equal perceived EV or equal implementation costs, people are more likely to get vaccinated in response to lotteries with very high payoffs than to small certain cash payout or slightly higher-probability, more moderate payoffs. In particular, given the potential appeal of official state lottery tickets, we will include the promise of Pennsylvania scratch-off lottery tickets among our study arms. We further hypothesize that lotteries will outperform simple reminders (encouraging respondents to get the flu shot at their upcoming appointment) and the standard of care, representing the ambient healthcare system and public health campaigns to increase vaccination. The primary outcome will be whether the patient was vaccinated at the scheduled appointment, as recorded in the EHR. A secondary outcome will be flu vaccination within 7 days of receiving the intervention. Additional secondary outcomes will include pre-commitment to get the shot and vaccination during the subsequent flu season.