Electronic nudges for sustained influenza vaccination uptake in older adults: the nationwide randomized NUDGE-FLU-2 trial.

Digital letter interventions have proven effective in increasing influenza vaccination rates. In this trial, we sought to further refine these strategies and investigated whether the effectiveness of the strategies could be sustained across consecutive influenza seasons. We enrolled all eligible Danish citizens 65 years of age or older in a nationwide registry-based randomized implementation trial during the 2023-2024 influenza season. Households of participants were randomly assigned in a 2.45:1:1:1:1:1:1 ratio to usual care or six different behaviorally informed electronic letter-based nudges delivered before the influenza vaccination period. The primary endpoint was receipt of influenza vaccination. Statistical analyses accounted for household-level clustering. A total of 881,373 participants (mean age 74.1 ± 6.5 years, 52.1% female) were randomized across 649,487 households. The primary endpoint was met; influenza vaccination rates were higher in the pooled intervention letter group compared to usual care (76.32% versus 76.02%; difference, 0.31 percentage points; 99.29% confidence interval, 0.00-0.61; P = 0.007). Although no individual letter significantly increased influenza vaccination rates, the directionality of effect was consistent across all letters. Effectiveness was particularly pronounced in participants who had not received influenza vaccination during the preceding season (P