Electronic Nudges and Influenza Vaccination Among Patients With a History of Myocardial Infarction

Importance Influenza vaccination in patients with acute myocardial infarction (AMI) reduces major adverse cardiac events and is strongly recommended in clinical practice guidelines. Effective strategies to improve vaccination are needed in these high-risk patients. Objective To evaluate whether electronically delivered behavioral nudges improve influenza vaccine uptake in patients with AMI across 3 nationwide implementation randomized clinical trials (RCTs). Design, Setting, and Participants Nationwide Utilization of Danish Government Electronic Letter System for Increasing Influenza Vaccine Uptake (NUDGE-FLU), Nationwide Utilization of Danish Government Electronic Letter System for Confirming the Effectiveness of Behavioral Nudges in Increasing Influenza Vaccine Uptake Among Older Adults (NUDGE-FLU-2), and Nationwide Utilization of Danish Government Electronic Letter System for Increasing Influenza Vaccine Uptake Among Adults With Chronic Disease (NUDGE-FLU-CHRONIC) were RCTs conducted during the 2022 to 2023 and 2023 to 2024 influenza seasons in Denmark. Participants were randomized to either usual care or various behaviorally informed, electronically delivered, letter-based nudges. In a prespecified participant-level pooled meta-analysis, interaction of AMI status on the effects of letter-based nudges vs usual care was examined. Pooled treatment effects were estimated using binomial regression models with identity link, adjustment for trial, and 2-way clustered SEs at the household and participant levels. Effect modification by recency of AMI as a continuous variable was assessed using restricted cubic spline modeling in NUDGE-FLU-CHRONIC. Interventions Behaviorally informed, electronically delivered, letter-based nudges or usual care. Main Outcome and Measures The primary end point was influenza vaccination receipt. Results Of 2 146 124 individual randomizations (mean [SD] age, 71.1 [11.6] years; 1 114 725 female [51.9%]) across all 3 trials, 59 458 (2.8%) had a history of AMI. Improvement in vaccine uptake was similar in patients with vs without a history of AMI who received any nudge letter compared with usual care (+1.81 vs +1.32 percentage points; P for interaction by AMI status = .09). A letter highlighting the cardiovascular benefits of vaccination (ie, cardiovascular-gain frame) resulted in larger improvements in vaccine uptake among patients with (vs without) a history of AMI (+3.91 vs +2.03 percentage points; P for interaction by AMI status = .002). Among patients with AMI, the benefits of the cardiovascular-gain frame letter were more pronounced in those not vaccinated in the prior season (+13.7 vs +1.48 percentage points; P for interaction <.001). Among younger participants with chronic disease, the cardiovascular-gain frame letter was particularly effective in patients with more recent AMI ( P for interaction by continuous recency of AMI <.001). Conclusions and Relevance Across 3 nationwide RCTs of Danish citizens, messaging emphasizing the cardiovascular benefits of vaccination improved influenza vaccination uptake, with greater benefits observed in patients with a history of AMI. This low-cost, scalable implementation strategy should be considered to encourage influenza vaccination in high-risk patients. Trial Registration ClinicalTrials.gov Identifiers: NCT05542004 , NCT06030726 , NCT06030739