Influenza (the “flu”) infects the majority of Americans every year and results in more than 200,000 hospitalizations, between 3,000 and 49,000 deaths (90% of which are in adults ≥65 years), and $87 billion in direct and indirect spending. The flu vaccine reduces the likelihood of infection by more than 60%, yet only 4 in 10 Americans actually get immunized. This large study conducted in partnership with the Center for Medicare and Medicaid Services and the National Vaccines Program Office, randomized 228,000 Medicare beneficiaries to one of five versions of letters intended to motivate vaccination, including versions with an implementation intention prompt and an enhanced active choice implementation prompt. All of the letters significantly increased vaccination compared with no letter although there was no difference across the 4 letters tailored with behavioral science techniques.