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 of 228,000 Medicare beneficiaries is evaluating whether patients can be engaged, at scale, by using behavioral economics and embedding “nudges” into mailings about influenza language to promote flu vaccinations.