Yearly influenza vaccination is strongly recommended for older adults and patients with chronic diseases including cardiovascular disease (CVD); however, vaccination rates remain suboptimal, particularly among younger patients. Electronic letters incorporating behavioral nudges are highly scalable public health interventions which can potentially increase vaccination, but further research is needed to determine the most effective strategies and to assess effectiveness across different populations. The purpose of NUDGE-FLU-CHRONIC and NUDGE-FLU-2 are to evaluate the effectiveness of electronic nudges delivered via the Danish governmental electronic letter system in increasing influenza vaccination among patients with chronic diseases and older adults, respectively.Both trials are designed as pragmatic randomized implementation trials enrolling all Danish citizens in their respective target groups and conducted during the 2023/2024 influenza season. NUDGE-FLU-CHRONIC enrolls patients aged 18-64 years with chronic diseases. NUDGE-FLU-2 builds upon the NUDGE-FLU trial conducted in 2022/2023 and aims to expand the evidence by testing both previously successful and new nudges among adults ≥65 years during a subsequent influenza season. Persons with exemptions from the electronic letter system are excluded from both trials. In both trials, participants are randomized in a 2.45:1:1:1:1:1:1 ratio to either receive no electronic letter (usual care) or to receive one of 6 different behaviorally informed electronic letters. NUDGE-FLU-CHRONIC has randomized 299,881 participants with intervention letters delivered on September 24, 2023, while NUDGE-FLU-2 has randomized 881,373 participants and delivered intervention letters on September 13, 2023. Follow-up is currently ongoing. In both trials, the primary endpoint is receipt of influenza vaccination on or before January 1, 2024, and the secondary endpoint is time to vaccination. Clinical outcomes including respiratory and cardiovascular hospitalizations, all-cause hospitalization, and mortality are included as prespecified exploratory endpoints. Prespecified individual-level pooled analyses will be conducted across NUDGE-FLU, NUDGE-FLU-CHRONIC, and NUDGE-FLU-2.NUDGE-FLU-CHRONIC is the first nationwide randomized trial of electronic nudges to increase influenza vaccination conducted among 18-64-year-old high-risk patients with chronic diseases. NUDGE-FLU-2 will provide further evidence on the effectiveness of electronic nudges among older adults ≥65 years. Collectively, the NUDGE-FLU trials will provide an extensive evidence base for future public health communications.NUDGE-FLU-CHRONIC: Clinicaltrials.gov: NCT06030739, registered September 11, 2023, https://clinicaltrials.gov/study/NCT06030739. NUDGE-FLU-2: Clinicaltrials.gov: NCT06030726, registered September 11, 2023, https://clinicaltrials.gov/study/NCT06030726.