Key Points
Question
Does high-dose inactivated influenza vaccine (HD-IIV) provide additional protection against cardiovascular events in the general older population compared with standard-dose inactivated influenza vaccine (SD-IIV)?
Findings
In this prespecified secondary analysis of the DANFLU-2 randomized clinical trial including 332 438 adults 65 years or older individually randomized to HD-IIV or SD-IIV, the incidence of cardiorespiratory hospitalization was 2.25% in the HD-IIV group and 2.38% in the SD-IIV group. The difference was driven primarily by a lower incidence of cardiovascular hospitalization (1.30% vs 1.40%).
Meaning
The results suggest additional effectiveness with HD-IIV compared with SD-IIV against cardiovascular outcomes but should be interpreted as exploratory findings in the setting of a large randomized clinical trial with a neutral primary outcome.
This secondary analysis of a randomized clinical trial investigates the relative vaccine effectiveness of high-dose vs standard-doze inactive influenza vaccine against cardiovascular outcomes in the general older adult population in Denmark.
Abstract
Importance
The high-dose inactivated influenza vaccine (HD-IIV) has demonstrated superior protection against laboratory-confirmed influenza infection vs standard-dose IIV (SD-IIV); however, data regarding its effectiveness against cardiovascular (CV) outcomes are mainly from observational studies or specific high-risk groups.
Objective
To investigate the relative vaccine effectiveness (rVE) of HD-IIV vs SD-IIV against CV outcomes in the general older adult population in Denmark.
Design, Setting, and Participants
This was a prespecified secondary analysis of DANFLU-2, a pragmatic, open-label, individually randomized clinical trial (RCT) using nationwide administrative health registries in Denmark during the 2022/2023 to 2024/2025 influenza seasons. Older adults (age ≥65 years) were eligible for inclusion regardless of comorbidity. The trial design specified that if the primary end point was neutral, no hypothesis testing would be performed for secondary or exploratory end points. Data were analyzed from June 29 to August 12, 2025.
Interventions
Individual-level 1:1 randomization to HD-IIV or SD-IIV. Participants re-enrolling in additional seasons were rerandomized.
Main Outcomes and Measures
Severe CV outcomes were prespecified secondary and exploratory end points in the trial, occurring from 14 days after vaccination through May 31 the following year.
Results
A total of 332 438 participants (170 900 [51.4%] male; mean [SD] age, 73.7 [5.8] years) were randomized (166 218 to HD-IIV and 166 220 to SD-IIV), of whom 91 026 (27.4%) had a history of CV disease. HD-IIV did not significantly reduce the trial’s primary end point of hospitalization for influenza or pneumonia. The incidence of hospitalization for any cardiorespiratory disease was lower in the HD-IIV group than the SD-IIV group (rVE, 5.7% [95% CI, 1.4% to 9.9%]; absolute difference, −0.13 [95% CI, −0.24 to −0.03] percentage points), and rVE did not differ by history of CV disease compared with no CV disease at baseline. Hospitalization for any CV disease occurred in fewer participants in the HD-IIV group than the SD-IIV group (rVE, 7.5% [95% CI, 1.5% to 12.5%]; absolute difference, −0.10 [95% CI, −0.18 to −0.02] percentage points) as did hospitalization for heart failure (rVE, 19.5% [95% CI, 3.3% to 33.1%]; absolute difference, −0.03 [95% CI, −0.06 to −0.01] percentage points).
Conclusions and Relevance
This study found reduced incidence of cardiorespiratory hospitalization among those who received HD-IIV vs SD-IIV, driven by a lower incidence of CV hospitalizations, and particularly heart failure hospitalizations. These differences should be interpreted as exploratory findings in the setting of a large RCT with a neutral primary outcome.
Trial Registration
ClinicalTrials.gov Identifier: NCT05517174
Introduction
Influenza is associated with an increased risk of adverse cardiovascular (CV) events, including atherosclerotic events, heart failure (HF) events, and atrial fibrillation.1,2,3 In recent years, increasing evidence has supported a protective effect of influenza vaccination against these outcomes, primarily among patients with established CV disease (CVD).4,5,6,7 The benefit was finally demonstrated in a randomized clinical trial setting in the Influenza Vaccination After Myocardial Infarction trial,8 published in 2021, in which a standard-dose inactive influenza vaccine (SD-IIV) reduced the risk of a composite of all-cause death, myocardial infarction (MI), or stent thrombosis by 28% compared with placebo among individuals with recent MI or high-risk stable coronary artery disease. The collective evidence has led to inclusion of influenza vaccination as a high-level recommendation in clinical practice guidelines for patients with acute coronary syndromes.9 However, SD-IIVs often do not elicit a sufficient immune response, particularly among older individuals and those with preexisting conditions, including CVD, resulting in suboptimal vaccine effectiveness.10,11,12 The high-dose IIV (HD-IIV) was developed to provide increased protection among older adults and has demonstrated 24.2% superior protection against laboratory-confirmed influenza infection compared with SD-IIV.13 However, in the Influenza Vaccine to Effectively Stop Cardio Thoracic Events and Decompensated Heart Failure trial (INVESTED),14 recruiting a high-risk population of 5260 patients aged at least 18 years with a recent MI or HF hospitalization, HD-IIV did not reduce the risk of a composite of all-cause death or cardiopulmonary hospitalization compared with SD-IIV.14 An important opportunity remains for further data on CV outcomes in a broader randomized general population and a wider spectrum of patients with CVD. Therefore, we conducted a prespecified secondary analysis of the large-scale, individually randomized Pragmatic Randomized Trial to Evaluate the Effectiveness of High-Dose Quadrivalent Influenza Vaccine vs Standard-Dose Quadrivalent Influenza Vaccine in Older Adults (DANFLU-2) trial15 to investigate the relative vaccine effectiveness (rVE) of HD-IIV compared with SD-IIV against CV outcomes in a broad population sample of adults aged 65 years and older.
Methods
Study Design and Participants
This was a prespecified secondary analysis of DANFLU-2, a pragmatic, registry-based, open-label, active-controlled, individually randomized clinical trial of HD-IIV vs SD-IIV conducted in Denmark during the 2022/2023, 2023/2024, and 2024/2025 influenza seasons. DANFLU-2 was approved as a low-intervention clinical trial by the Medical Research Ethics Committees in Denmark and the Danish Medicines Agency. Approval for data collection and storage was granted by the data authority in the Capital Region of Denmark. All participants provided informed consent. The trial design and primary results have been published separately.15,16 The trial’s hierarchical test procedure specified that if the primary end point was neutral, secondary or exploratory end points were to be reported descriptively only, without hypothesis testing. The trial protocol and statistical analysis plan are available in Supplement 1. This report follows the Consolidated Standards of Reporting Trials (CONSORT) reporting guideline.
The study was designed to evaluate the rVE of HD-IIV vs SD-IIV in a broad, older general population sample and therefore enrolled adults aged at least 65 years regardless of comorbidity. The trial had no formal exclusion criteria, and participants were only required to attend 1 in-person study visit for randomization and vaccination. Study visits occurred at vaccination sessions organized by the private vaccination provider Danske Lægers Vaccinations Service/European LifeCare Group at more than 500 unique locations, separately from the Danish governmental vaccination program. Participants were recruited primarily through electronic invitation letters distributed via the Danish governmental electronic letter system17 to up to 1 000 000 randomly selected Danish citizens aged 65 years or older each season. Participants signed up through a website or via phone and were able to provide informed consent online prior to their study visit with no mandatory requirement of real-time contact with study staff; in-person consent was also available on site.
Interventions
Participants were randomly allocated 1:1 at the individual level to either HD-IIV or SD-IIV. Allocation was implemented using a tablet application linked to a centralized unstratified randomization algorithm. Participants were allowed to re-enroll in subsequent seasons and were then rerandomized; such participants were treated as unique observations for statistical purposes.18 The study was open-label, but bias was anticipated to be minimal as the intervention (HD-IIV) was not available elsewhere in Denmark, trial personnel were not involved in treatment of participants or coding of end point data, and severe hospitalization–based end points were not expected to be affected by knowledge of treatment assignment.
Participants received a single dose of either quadrivalent HD-IIV (2022/2023 and 2023/2024 seasons: Fluzone High-Dose Quadrivalent or Efluelda; 2024/2025 season: Efluelda Tetra [Sanofi]) or quadrivalent SD-IIV (VaxigripTetra [Sanofi]), administered intramuscularly. At 60 µg of hemagglutinin antigen for each strain, HD-IIV contains 4 times as much antigen as SD-IIV (15 µg per strain). Prior research demonstrating noninferiority between quadrivalent and trivalent HD-IIV19 ensures that the results of this trial, which used quadrivalent vaccines only, remain applicable in an era where the world is returning to trivalent IIV formulations.20 Coadministration with other seasonal vaccines as per local guidelines was allowed.
Data Collection
The trial used passively collected health data obtained from nationwide Danish administrative health registries for assessment of baseline characteristics and end point events and to survey for potential serious adverse events.21 Centralized oversight and standardized, automated data capture ensures high data quality of these registries, which are used extensively for both observational and interventional research.21,22,23,24 A unique personal identifier assigned to each Danish legal resident at birth or emigration ensures unambiguous data linkage. Baseline data and end points were ascertained using prespecified definitions (eTables 1-4 and eAppendix in Supplement 2). As prespecified in the trial protocol, history of CVD was defined as a history of at least 1 of the following conditions: ischemic heart disease, pulmonary embolism, valvular disease, cardiomyopathy, arrhythmia, cardiac arrest, HF, cerebrovascular disease, peripheral vascular disease, or congenital heart disease.
End Points
Participants were followed for the occurrence of end point events from 14 days postvaccination through May 31 the following year. Only first events were considered for each end point. Hospitalization end points were ascertained using International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) codes; in the Danish health system, coding is generally performed by the treating physicians. As prespecified, hospitalization-based events with an associated COVID-19 ICD-10 discharge diagnosis code (B34.2, B97.2) were not considered eligible as end points as to improve outcome specificity. The trial’s primary end point was hospitalization for influenza or pneumonia, for which the results have been reported separately HD-IIV did not significantly reduce the incidence of the primary end point compared with SD-IIV (rVE, 5.9% [95.2% CI, −2.1% to 13.4%]; P = .14).16 In this secondary analysis, we further assessed first secondary end point of hospitalization for any cardiorespiratory disease across major CV subgroups. In addition, we assessed the rVE of HD-IIV vs SD-IIV against the prespecified end points of hospitalization for any CVD, respiratory disease, MI, HF, atrial fibrillation, stroke, and major adverse CV events, defined as a composite of MI, stroke, or all-cause death. The effects of HD-IIV vs SD-IIV against these end points were also assessed according to whether the condition was present at baseline (eg, MI assessed according to history of ischemic heart disease) to estimate separately the effects on worsening of preexisting disease vs new-onset disease. Both components of the hospitalization for any cardiorespiratory disease end point (ie, hospitalization for any CVD and hospitalization for any respiratory disease) were tested according whether or not the participants had a history of CVD. Prespecified sensitivity analyses restricted the follow-up period to the peak of the influenza seasons, defined as the period from the first week to the last week (inclusive) during which the influenza test positivity rate in national surveillance data, with separate analyses for the 55th percentile or higher and 75th percentile or higher.
Statistical Analysis
At a projected sample size of 339 700 participants, the trial would have an estimated 90% power to detect a 12.50% rVE with HD-IIV vs SD-IIV against the trial’s primary end point, assuming an attack rate of 0.74% in the SD-IIV group. In addition, the trial would have 90% power to detect a 7.50% rVE with HD-IIV vs SD-IIV against the first secondary end point of hospitalization for any cardiorespiratory disease, assuming an attack rate of 2.10% in the SD-IIV group.25
The main analyses were performed using the intention-to-treat principle, with rVE calculated as 1 minus the crude relative risk of the end point, expressed as a percentage. We computed 95% CIs using the Clopper-Pearson method.26 No accounting for censoring was prespecified for rVE calculation, as censoring was expected to occur very rarely. For subgroup analyses, we calculated rVE for each subgroup stratum. Interaction P values were estimated using the Cochran-Mantel-Haenszel test for homogeneity. Additional supportive analyses were conducted including an as-treated analysis, an analysis including end point events occurring within 14 days after vaccination, and an analysis accounting for potential cross-season correlation in participants enrolled in more than 1 season.
In light of the trial not finding a significant different in the primary end point, no hypothesis testing was conducted for this report. The 95% CIs are not adjusted for multiplicity and therefore should not be used to infer treatment effects. P values were 2-sided, and statistical significance was set at P ≤ .05. Statistical analysis was performed using SAS software version 9.4 (SAS Institute) and Stata MP version 19.5 (StataCorp). Data were analyzed from June 29 to August 12, 2025.
Results
Participants
A total of 332 438 participants (170 900 [51.4%] male; mean [SD] age, 73.7 [5.8] years) were randomized in the trial during the 2022/2023 (19 699 participants), 2023/2024 (160 451 participants), and 2024/2025 (152 288 participants) influenza seasons, with 166 218 participants randomized to HD-IIV and 166 220 participants randomized to SD-IIV. Recruitment, allocation, and intervention administration in the trial are illustrated in the eFigure in Supplement 2. A history of CVD was present in 91 026 participants (27.4%), including 31 112 participants (9.4%) with ischemic heart disease, 9405 participants (2.8%) with a history of MI, 10 410 participants (3.1%) with HF, and 34 085 participants (10.3%) with atrial fibrillation (Table 1). Participants with a history of CVD were older, more likely to be male, and had a higher prevalence of non-CV comorbidity, including cancer and chronic lung disease, compared with those with no history of CVD. Among those with a history of CVD, a total of 17 326 participants (19.0%) had diabetes and 22 281 participants (24.5%) had chronic kidney disease. Baseline characteristics were well-balanced across randomized groups among those with and without CVD (Table 1).
Table 1. Baseline Characteristics According to Randomization Group Among Participants With and Without a History of CVD.
| Characteristic | Participants, No. (%)a | |||
|---|---|---|---|---|
| CVD | No CVD | |||
| HD-IIV (n = 45 688) | SD-IIV (n = 45 338) | HD-IIV (n = 120 528) | SD-IIV (n = 120 875) | |
| Age, mean (SD), y | 75.3 (6.1) | 75.3 (6.2) | 73.1 (5.5) | 73.1 (5.5) |
| Sex | ||||
| Female | 16 727 (36.6) | 16 590 (36.6) | 64 052 (53.1) | 64 163 (53.1) |
| Male | 28 961 (63.4) | 28 748 (63.4) | 56 476 (46.9) | 56 712 (46.9) |
| CVD | ||||
| Ischemic heart disease | 15 666 (34.3) | 15 446 (34.1) | NA | NA |
| Myocardial infarction | 4730 (10.4) | 4675 (10.3) | NA | NA |
| Atrial fibrillation | 17 076 (37.4) | 17 009 (37.5) | NA | NA |
| Heart failure | 5201 (11.4) | 5209 (11.5) | NA | NA |
| Valvular disease | 7639 (16.7) | 7484 (16.5) | NA | NA |
| Cerebrovascular disease | 8371 (18.3) | 8010 (17.7) | NA | NA |
| Peripheral vascular disease | 1512 (3.3) | 1467 (3.2) | NA | NA |
| Diabetes | 8655 (18.9) | 8671 (19.1) | 13 274 (11.0) | 13 281 (11.0) |
| Chronic kidney disease | 11 134 (24.4) | 11 147 (24.6) | 12 168 (10.1) | 12 339 (10.2) |
| Cancer | 7651 (16.7) | 7423 (16.4) | 15 271 (12.7) | 15 573 (12.9) |
| Chronic lung disease | 5818 (12.7) | 5672 (12.5) | 7757 (6.4) | 7905 (6.5) |
| Chronic obstructive pulmonary disease | 3259 (7.1) | 3123 (6.9) | 3540 (2.9) | 3566 (3.0) |
| Immunodeficiency | 2570 (5.6) | 2502 (5.5) | 4617 (3.8) | 4626 (3.8) |
| Influenza vaccination during preceding season | 44 616 (97.7) | 44 315 (97.7) | 116 342 (96.5) | 116 784 (96.6) |
| Coadministration with COVID-19 vaccineb | 27 988 (61.3) | 27 799 (61.3) | 74 254 (61.6) | 74 682 (61.8) |
| COVID-19 vaccine during same season | 44 031 (96.4) | 43 712 (96.4) | 116 405 (96.6) | 116 720 (96.6) |
Abbreviations: CVD, cardiovascular disease; HD-IIV, high-dose inactive influenza vaccine; SD-IIV, standard-dose inactive influenza vaccine.
Baseline characteristics were sourced from nationwide administrative health registries using prespecified definitions (eTables 1-3 and eAppendix in Supplement 2). A total of 9 participants withdrew consent prior to registry data linkage, and these participants therefore do not contribute any baseline data other than age and sex.
COVID-19 vaccine coadministration was defined as receiving COVID-19 vaccination on the same day as the study vaccine.
CV Outcomes in the Overall Trial Population
The incidence of hospitalization for any cardiorespiratory disease was 3735 participants (2.25%) in the HD-IIV group and 3962 participants (2.38%) in the SD-IIV group (rVE, 5.7% [95% CI, 1.4% to 9.9%]; absolute difference, −0.13 [95% CI, −0.24 to −0.03] percentage points) (Table 2).16 The incidence of hospitalization for any CVD was 2156 participants (1.30%) in the HD-IIV group and 2323 participants (1.40%) in the SD-IIV group (rVE, 7.2% [95% CI, 1.5% to 12.5%]; absolute difference, −0.10 [95% CI, −0.18 to −0.02] percentage points) and hospitalization for any respiratory disease occurred in 1677 participants (1.01%) and 1742 participants (1.05%), respectively (rVE, 3.7% [95% CI, −3.0% to 10.0%]; absolute difference, −0.04 [95% CI, −0.11 to 0.03] percentage points). Of the investigated subsets of hospitalization for any CVD, the rVE point estimate for hospitalization for HF particularly favored HD-IIV compared with SD-IIV (rVE, 19.5% [95% CI, 3.3% to 33.1%]; absolute difference, −0.03 [95% CI, −0.06 to −0.01] percentage points). The observed absolute difference in risk of cardiorespiratory hospitalization corresponded to a number needed to vaccinate with HD-IIV vs SD-IIV of 732 (95% CI, 425 to 3010) to prevent 1 additional event. An as-treated analysis, as well as analyses including events occurring within the first 14 days after randomization and accounting for potential cross-season correlation, yielded similar results for all end points (eTables 5-7 in Supplement 2). In a prespecified sensitivity analysis, rVE against hospitalization for any cardiorespiratory disease and hospitalization for HF was increased during periods with substantial influenza circulation (eTable 8 in Supplement 2).
Table 2. Cardiovascular Outcomes in the Overall Trial Population.
| End point | Participants, No. (%) | Relative vaccine effectiveness, % (95% CI) | Absolute difference, percentage points (95% CI) | No. needed to vaccinate (95% CI)a | |
|---|---|---|---|---|---|
| HD-IIV (n = 166 218) | SD-IIV (n = 166 220) | ||||
| Hospitalization for any cardiorespiratory disease | 3735 (2.25) | 3962 (2.38) | 5.7 (1.4 to 9.9) | −0.14 (−0.24 to −0.03) | 732 (425 to 3010) |
| Exploratory end points | |||||
| Hospitalization | |||||
| Any CVD | 2156 (1.30) | 2323 (1.40) | 7.2 (1.5 to 12.5) | −0.10 (−0.18 to −0.02) | 995 (572 to 4638) |
| Any respiratory disease | 1677 (1.01) | 1742 (1.05) | 3.7 (−3.0 to 10.0) | −0.04 (−0.11 to 0.03) | 2558 (418 to ∞) |
| Myocardial infarction | 265 (0.16) | 265 (0.16) | 0.0 (−19.0 to 16.0) | 0.0 (−0.03 to 0.03) | NA |
| Heart failure | 214 (0.13) | 266 (0.16) | 19.5 (3.3 to 33.1) | −0.03 (−0.06 to −0.01) | 3197 (1887 to 18 828) |
| Atrial fibrillation | 757 (0.46) | 790 (0.48) | 4.2 (−6.0 to 13.4) | −0.02 (−0.07 to 0.03) | 5038 (1572 to ∞) |
| Stroke | 458 (0.28) | 485 (0.29) | 5.6 (−7.5 to 17.1) | −0.02 (−0.05 to 0.02) | 6158 (2008 to ∞) |
| MACEb | 973 (0.59) | 1000 (0.60) | 2.7 (−6.4 to 11.0) | −0.02 (−0.07 to 0.04) | 6159 (1510 to ∞) |
| MACE (alternate definition)c | 1158 (0.70) | 1224 (0.74) | 5.4 (−2.6 to 12.8) | −0.04 (−0.10 to 0.02) | 2519 (1063 to ∞) |
| Cardiovascular death | 291 (0.18) | 279 (0.17) | −4.3 (−23.4 to 11.8) | 0.01 (−0.02 to 0.04) | NA |
Abbreviations: CVD, cardiovascular disease; HD-IIV, high-dose inactive influenza vaccine; MACE, major adverse cardiovascular event; NA, not available; SD-IIV, standard-dose inactive influenza vaccine.
Number of individuals needed to be vaccinated with HD-IIV instead of SD-IIV to prevent 1 additional event.
Defined as a composite of hospitalization for myocardial infarction, hospitalization for stroke, or cardiovascular death.
Also included heart failure hospitalization.
Subgroup Analyses
The rVE point estimates for hospitalization for any cardiorespiratory disease were consistent across examined subgroups (Figure 1), including no suggested effect modification according to the presence or absence of CVD at baseline (CVD at baseline: rVE, 5.8% [95% CI, 0.0% to 11.4%]; no CVD at baseline: rVE, 6.2% [95% CI, −0.3% to 12.4%]; P for interaction = .92). Similarly consistent point estimates were observed for hospitalization for any CVD (CVD at baseline: rVE, 6.7% [95% CI, −0.7% to 13.6%]; no CVD at baseline: rVE, 8.7% [95% CI, −0.3% to 16.9%]; P for interaction = .72) (Figure 2).
Figure 1. Relative Vaccine Effectiveness (rVE) of High-Dose vs Standard-Dose Influenza Vaccine Against Hospitalization for Cardiorespiratory Disease Across Prespecified Subgroups.
Figure 2. Relative Vaccine Effectiveness (rVE) of High-Dose vs Standard-Dose Influenza Vaccine Against Additional Cardiorespiratory Outcomes According to Established Disease at Baseline.
In addition, the point estimates for rVE against hospitalization for HF with HD-IIV compared with SD-IIV were consistent, regardless of whether the outcome represented worsening of preexisting disease or new-onset disease (HF at baseline: rVE, 17.5% [95% CI, −11.2% to 39.0%]; no HF at baseline: rVE, 20.7% [95% CI, −0.4% to 37.5%]; P for interaction = .83) (Figure 2). Additional results from similar analyses of the remaining examined outcomes are shown in Figure 2.
Discussion
In this prespecified secondary analysis of the largest individually randomized clinical trial of influenza vaccines conducted among 332 438 adults aged 65 years or older, the incidence of cardiorespiratory hospitalization was lower with HD-IIV compared with SD-IIV in older adults, although the findings should be considered hypothesis-generating in the context of the trial’s neutral primary results. The observed rVE point estimates for cardiorespiratory hospitalization were consistent across major CV subgroups and were driven by a difference in CV hospitalizations (rVE, 7.2% [95% CI, 1.5% to 12.5%]). The rVE point estimate for HD-IIV vs SD-IIV against HF hospitalizations approached 20% and remained consistent regardless of the presence or absence of HF at baseline. The observed differences between HD-IIV and SD-IIV in CV events was more pronounced during periods with high levels of influenza circulation, supporting that the observed differences were driven by events directly related to influenza. Similar rates of serious adverse events have previously been reported between the HD-IIV and SD-IIV groups.16
Although the observed absolute differences in outcomes may seem small, they could still represent a potentially meaningful clinical benefit. As vaccines aim to prevent disease, effect sizes cannot be compared with those observed with standard medications for specific treatment of established disease, and in the case of this trial, HD-IIV was compared with an already-effective active comparator (SD-IIV) in a nonenriched, broad, older general population sample. Influenza vaccination is already recommended for a broad range of individuals, and the World Health Organization estimates that approximately 850 million doses are administered yearly, with enhanced vaccines representing only 10% of these.27 At an estimated number needed to vaccinate of 732, replacing 765 million administered SD-IIVs with HD-IIVs could hypothetically result in more than 1 million additional averted cardiorespiratory hospitalizations worldwide each year. Moreover, the results of this analysis suggest consistent rVE point estimates across risk groups, indicating that higher absolute effect sizes could be attained in subpopulations with higher baseline risk (eg, the observed difference in absolute risk of cardiorespiratory hospitalization approached 1% among patients with HF).
While similar findings for the rVE of HD-IIV vs SD-IIV against cardiorespiratory hospitalizations in older adults have been reported previously, the scale of DANFLU-2 by far exceeds that of prior research in the field. In a 2023 meta-analysis,28 the rVE of HD-IIV vs SD-IIV against cardiorespiratory hospitalization in the general older adult population was estimated as 12.2% (95% CI, 5.6%-18.3%), based on a total of 97 474 participants enrolled in 3 prior randomized clinical trials, 1 of which was cluster-randomized.23,29,30 At 332 438 individually randomized participants, DANFLU-2 enrolled more than 3 times as many participants as those included in the meta-analysis estimate.28
In the INVESTED trial,14 the only prior randomized trial of HD-IIV vs SD-IIV specifically powered for a cardiorespiratory outcome to our knowledge, HD-IIV did not reduce the risk of all-cause death or cardiopulmonary hospitalization (hazard ratio, 1.06 [95% CI, 0.97-1.17]), and both total CV hospitalizations and HF hospitalizations were numerically higher in the HD-IIV group. However, the study population was very different in age and baseline characteristics compared with DANFLU-2, as the INVESTED trial14 enrolled a younger and very high-risk population: patients 18 years or older who had been hospitalized for MI within 12 months prior to baseline or for HF within 24 months prior to baseline and had at least 1 additional risk factor. Several potential reasons may explain the apparent disagreement between the INVESTED trial14 and the findings of this analysis. The INVESTED trial14 enrolled an enriched cohort of patients with high risk, including history of MI and HF, for whom influenza may only contribute minor excess risk compared with numerous other driving factors. While just the number of patients with HF enrolled in DANFLU-2 (n = 10 410) was nearly double that of the entire INVESTED trial enrollment (N = 5260), the DANFLU-2 HF subgroup likely represents a much broader range of patients with HF, including patients at lower baseline risk of hospitalization in whom influenza may represent a proportionally larger hazard. In the INVESTED trial,14 only 18 of 2878 total cardiopulmonary hospitalizations (0.6%) were judged as influenza-related, while in DANFLU-2, a much larger fraction of cardiorespiratory hospitalizations was due to influenza (280 of 7697 [3.6%]), again suggesting that influenza may have been a more prominent driver of outcomes in DANFLU-2 compared with INVESTED.
We observed rVE of 19.5% (95% CI, 3.3%-33.1%) against HF hospitalizations with HD-IIV vs SD-IIV, with the point estimate consistent regardless of whether HF was present at baseline or not, although it should be noted that the absolute difference was small. To our knowledge, the only other trial of HD-IIV vs SD-IIV to report HF hospitalizations as an outcome besides the INVESTED trial14 was a cluster-randomized trial enrolling US nursing home residents aged 65 years or older that, contrary to other findings, found no reduction in the outcome (adjusted hazard ratio, 0.96 [95% CI, 0.80-1.15]).30 Most trials of influenza vaccine vs placebo or no vaccine in relation to CV outcomes have focused on atherosclerotic CVD, with meta-analyses demonstrating substantial reductions in major CV events.31,32 To our knowledge, only 1 trial has been powered for an HF-related primary outcome: the Influenza Vaccine to Prevent Adverse Vascular Events trial33 randomized 5129 patients with HF in low- and middle-income countries to SD-IIV or placebo and found no difference in either of 2 co–primary outcomes, with 1 outcome including HF hospitalization in a recurrent-events composite alongside CV death, nonfatal MI, and nonfatal stroke.33 No significant reduction in the HF hospitalization component was reported. The difference in HF hospitalization observed in DANFLU-2, particularly during peak influenza circulation and even among patients with preexisting HF, could represent a potential clinical benefit of HD-IIV beyond SD-IIV not demonstrated previously. While further validation is warranted, this finding merits consideration in clinical decision-making for high-risk CVD populations.
The DANFLU-2 trial represents a paradigm shift in evidence generation for the effectiveness of vaccines against severe outcomes. By combining criterion-standard individual-level randomization with pragmatic design elements, including digital recruitment, online informed consent, and comprehensive use of routine health data, this streamlined trial provides causal inference within a general-population vaccination setting, all while imposing minimal participation burden. This secondary analysis of DANFLU-2 demonstrates the utility of the large-scale data generated by the trial beyond its primary analysis: to deliver insights from secondary and exploratory analyses for cardiovascular outcomes and for high-risk populations (eg, patients with HF) with much larger sample sizes than seen in previous trials.
Limitations
This study has limitations. First, the trial did not meet its primary end point and the findings reported here should be considered hypothesis-generating; however, the trial was adequately powered to evaluate cardiorespiratory hospitalizations. These findings, while not subject to formal type I error control, provide potentially important clinical insights. Second, the trial was open-label, but risk of bias was considered low, as knowledge of treatment assignment was not expected to have a substantial effect on severe hospitalization–based events, and participants received clinical treatment by health care practitioners not involved in the trial. Third, assessment of end points and baseline conditions relied on national registry data rather than trial-initiated clinical assessment, which may introduce misclassification of outcomes. However, this would not be expected to be differentially distributed across randomization groups, and severe conditions, such as CV outcomes, are considered well-captured in the registries.34
Conclusions
In this prespecified secondary analysis of the DANFLU-2 randomized clinical trial in which the primary end point was not met, the incidence of cardiorespiratory hospitalization was lower among adults 65 years or older who received HD-IIV compared with SD-IIV, irrespective of previous history of CVD. The difference was driven by a lower incidence of CV hospitalizations, especially HF hospitalizations. The differences were augmented during periods with substantial influenza circulation. These findings should be considered hypothesis-generating in the context of the trial’s neutral primary results.
Trial Protocol and Statistical Analysis Plan
eFigure. CONSORT Diagram
eTable 1. Prespecified Definitions for Baseline Conditions
eTable 2. Prespecified Definitions for Medication Use at Baseline
eTable 3. Prespecified Definitions for Prior Vaccinations at Baseline
eTable 4. Prespecified Endpoint Definitions
eTable 5. As-Treated Analysis
eTable 6. Analysis of Cardiovascular Outcomes in the Overall Trial Population Including Endpoint Events Occurring Within 14 Days After Vaccination
eTable 7. Analysis Accounting for Potential Cross-Season Correlation in Participants Enrolled in >1 Season
eTable 8. Cardiovascular Outcomes in the Overall Trial Population According to Influenza Circulation Levels (Peak-Season Analysis)
eAppendix. Prespecified Definition of Immunosuppression
Data Sharing Statement
References
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Trial Protocol and Statistical Analysis Plan
eFigure. CONSORT Diagram
eTable 1. Prespecified Definitions for Baseline Conditions
eTable 2. Prespecified Definitions for Medication Use at Baseline
eTable 3. Prespecified Definitions for Prior Vaccinations at Baseline
eTable 4. Prespecified Endpoint Definitions
eTable 5. As-Treated Analysis
eTable 6. Analysis of Cardiovascular Outcomes in the Overall Trial Population Including Endpoint Events Occurring Within 14 Days After Vaccination
eTable 7. Analysis Accounting for Potential Cross-Season Correlation in Participants Enrolled in >1 Season
eTable 8. Cardiovascular Outcomes in the Overall Trial Population According to Influenza Circulation Levels (Peak-Season Analysis)
eAppendix. Prespecified Definition of Immunosuppression
Data Sharing Statement
