Preventive health behaviors like flu vaccination have important benefits, but compliance is poor, and the reasons are not fully understood. We conducted a large study across six colleges (N = 9358), with a methodology that offers an unusual opportunity to look at three potential factors: inattention to information, informed intentions to not comply, and problems following through on intentions. We also tested three interventions in an RCT. We find that inattention to information is not the primary driver of low take-up, while informed decisions to not get the vaccine, but also lack of follow-through, are important factors. A financial intervention increased take-up and had persistent, positive effects on intentions for vaccination in future years. Two low-cost “nudges” did not increase vaccination rates, although the peer endorsement nudge increased exposure to information, especially if aligned with social networks.