Influenza causes 36,000 U.S. deaths per year, but influenza immunization rates average just 28%. Behavioral "nudges" may increase the effectiveness of immunization reminder mailers at little or no added cost. Past psychology research has demonstrated that prompting people to form an implementation plan of the form, "When situation x arises, I will implement response y," increases attainment of desired goals because the desired behavior is linked to a concrete future moment. We study whether adding a planning prompt to a vaccination reminder mailer increases immunization rates.