Aims: Seasonal influenza vaccination is strongly recommended in patients with heart failure (HF). The NUDGE-FLU trial recently found two electronic behavioural nudging letter strategies – a letter highlighting potential cardiovascular benefits of vaccination and a repeated letter at day 14 –effective in increasing influenza vaccination in Denmark. The aims of this pre-specified analysis was to further examine vaccination patterns and effects of these behavioural nudges in patients with HF including potential off-target effects on guideline-directed medical therapy (GDMT) use. Methods and results: The nationwide NUDGE-FLU trial randomized 964 870 Danish citizens ≥65 years to usual care or nine different electronic nudging letter strategies. Letters were delivered through the official Danish electronic letter system. The primary endpoint was the receipt of an influenza vaccine; additional outcomes for this analysis included GDMT use. In this analysis, we also assessed influenza vaccination rates in the overall Danish HF population including those <65 years (n = 65 075). During the 2022–2023 season, influenza vaccination uptake was 71.6% in the overall Danish HF population but this varied considerably with only 44.6% uptake in those <65 years. A total of 33 109 NUDGE-FLU participants had HF at baseline. Vaccination uptake was higher among those on higher levels of baseline GDMT (≥3 classes: 85.3% vs. ≤2 classes: 81.9%; p < 0.001). HF status did not modify the effects of the two overall successful nudging strategies on influenza vaccination uptake (cardiovascular gain-framed letter: p_interaction = 0.37; repeated letter: p_interaction = 0.55). No effect modification was observed across GDMT use levels for the repeated letter (p_interaction = 0.88), whereas a trend towards attenuated effect among those on low levels of GDMT was observed for the cardiovascular gain-framed letter (p_interaction = 0.07). The letters had no impact on longitudinal GDMT use. Conclusions: Approximately one in four patients with HF did not receive influenza vaccination with a pronounced implementation gap in those <65 years where less than half were vaccinated. HF status did not modify the effectiveness of cardiovascular gain-framed and repeated electronic nudging letters in increasing influenza vaccination rates. No unintended negative effects on longitudinal GDMT use were observed. Clinical Trial Registration: ClinicalTrials.gov NCT05542004.