Background: In the NUDGE-FLU (Nationwide Utilization of Danish Government Electronic letter system for increasing inFLUenza vaccine uptake) trial, electronic letters incorporating cardiovascular (CV) gain-framing and repeated messaging increased influenza vaccination by approximately 1 percentage point. Objective: To evaluate the effects of the successful nudging interventions on downstream clinical outcomes. Design: Prespecified exploratory analysis of a nationwide randomized implementation trial. (ClinicalTrials. gov: NCT05542004) Setting: The 2022 to 2023 influenza season. Participants: 964 870 Danish citizens aged 65 years or older. Intervention: Usual care or 9 different electronically delivered behavioral nudging letters. Measurements: Cardiovascular, respiratory, and other clinical end points during follow-up from intervention delivery (16 September 2022) through 31 May 2023. Results: The analysis set included 691 820 participants. Hospitalization for pneumonia or influenza occurred in 3354 of 346 327 (1.0%) participants in the usual care group, 396 of 38 586 (1.0%) in the CV gain-framing group (hazard ratio [HR], 1.06 [95% CI, 0.95 to 1.18]; versus usual care), and 403 of 38 231 (1.1%) in the repeated letter group (HR, 1.09 [CI, 0.98 to 1.21]; versus usual care). In the usual care group, 44 682 (12.9%) participants were hospitalized for any cause, compared with 5002 (13.0%) in the CV gain-framing group (HR, 1.00 [CI, 0.97 to 1.03]; versus usual care) and 4965 (13.0%) in the repeated letter group (HR, 1.01 [CI, 0.98 to 1.04]; versus usual care). A total of 6341 (1.8%) participants died in the usual care group, compared with 721 (1.9%) in the CV gain-framing group (HR, 1.02 [CI, 0.94 to 1.10]; versus usual care) and 646 (1.7%) in the repeated letter group (HR, 0.92 [CI, 0.85 to 1.00]; versus usual care). Limitation: Prespecified but exploratory analysis, potential misclassification of events in routinely collected registry data, and results may not be generalizable to other health systems or countries with other racial compositions and/or cultural or societal norms. Conclusion: In a prespecified exploratory analysis, modest increases in influenza vaccination rates seen with electronic nudges did not translate into observable improvements in clinical outcomes. Seasonal influenza vaccination should remain strongly recommended.