BACKGROUND: Influenza contributes substantially to disease burden in individuals with heart failure (HF) and is an established trigger of cardiovascular and HF events. Standard-dose inactivated influenza vaccine (SD-IIV) is recommended for HF, though immune responses may be attenuated. High-dose inactivated influenza vaccine (HD-IIV) was developed to enhance immunogenicity, but its effectiveness compared with SD-IIV against hospitalization for influenza and cardiovascular disease by HF status remains uncertain. METHODS: This was a prespecified analysis of a pragmatic, prospective, individually randomized, open-label trial with registry-based end point-evaluation conducted in Denmark across the 2022/2023 to 2024/2025 influenza seasons. Citizens ≥65 years were randomized 1:1 to HD-IIV or SD-IIV. Outcomes included hospitalization for influenza-related illness, laboratory-confirmed influenza, any cardiovascular disease, cardio-respiratory disease, and HF, assessed by HF status. Effect of HD-IIV versus SD-IIV in reducing risk of outcomes assessed was expressed as risk ratios. RESULTS: The trial randomized 332 438 participants (48.6% female; mean age, 73.7±5.8 years), including 10 410 with HF at baseline (27.4% female; mean age, 76.0±6.3 years). Overall, HD-IIV was associated with a statistically significant lower incidence of hospitalization for influenza-related illness, laboratory-confirmed influenza, cardio-respiratory disease, cardiovascular disease, and HF compared with SD-IIV. In participants with HF, effect estimates were similar: risk ratio for influenza-related hospitalization was 0.48 (95% CI, 0.20-1.06; Pinteraction=0.64), for laboratory-confirmed influenza hospitalization 0.55 (95% CI, 0.29-1.02; Pinteraction=0.59), for cardio-respiratory hospitalization 0.89 (95% CI, 0.77-1.02; Pinteraction=0.34), for cardiovascular hospitalization 0.86 (95% CI, 0.72-1.02; Pinteraction=0.34), and for HF hospitalization 0.82 (95% CI, 0.61-1.11; Pinteraction=0.83). Findings were consistent across HF subgroups by disease duration, recency of hospitalization, most recent NT-proBNP (N-terminal pro-B-type natriuretic peptide), and presence of device therapy. CONCLUSIONS: In this prespecified exploratory analysis of the largest individually randomized influenza vaccine trial ever conducted, HD-IIV was associated with lower rates of influenza and cardiovascular hospitalizations compared with SD-IIV, with effect estimates similar across HF status at baseline and HF subgroups. REGISTRATION: URL: https://www.clinicaltrials.gov; Unique identifier: https://clinicaltrials.gov/study/NCT05517174.