Relative effectiveness of high-dose versus standard-dose quadrivalent influenza vaccine against hospitalizations and mortality according to frailty score: a post-hoc analysis of the DANFLU-1 trial

Abstract Background Frailty is a major risk factor for adverse cardiovascular events. Influenza vaccination has been shown to protect against cardiovascular fatal and nonfatal events. However, limited data exist on the added benefit of high-dose versus standard-dose influenza vaccination according to frailty status. Purpose We sought to assess the relative effectiveness of high-dose (QIV-HD) versus standard-dose (QIV-SD) quadrivalent influenza vaccination according to frailty status in elderly individuals. Methods This is a post-hoc analysis of the randomized controlled feasibility trial of QIV-HD versus QIV-SD conducted during the 2021–2022 influenza season in adults aged 65–79 years. We assessed the following prespecified outcomes: hospitalizations for pneumonia or influenza, cardio-respiratory hospitalizations, cardiovascular hospitalizations, all-cause hospitalizations and all-cause mortality. Level of frailty was defined according to the Hospital Frailty Risk Score based on ICD-10 codes for relevant comorbidities any time prior to vaccination. We divided the participants into two categories according to Frailty Score (FS): low frailty (0-4 points) and intermediate or high frailty (≥5 points). We analyzed outcomes as time to first event using Cox proportional hazards regression. Results Among 12,473 randomly assigned participants to QIV-HD versus QIV-SD (mean age 71.7 years, 47.1% female), 10,711 (85.9%) were categorized as having low frailty and 1,762 (14.1%) had intermediate or high frailty. In total, 38 individuals were hospitalized for pneumonia or influenza during follow-up, 219 were hospitalized for cardiorespiratory illness, 1055 were hospitalized due to any cause and 62 died during follow-up. FS significantly modified the effect of QIV-HD versus QIV-SD for all-cause mortality (P for interaction 0.02). In participants with low frailty, QIV-HD was associated with a lower risk of all-cause mortality (Figure 1: HR 0.26, 95% CI 0.13 to 0.55), whereas there was no evidence of effect among those with intermediate or high frailty (HR 1.68, 95% CI 0.66 to 4.26). QIV-HD versus QIV-SD was associated with a lower incidence of hospitalizations for pneumonia and influenza, this effect was consistent regardless of frailty status (P for interaction 0.88). QIV-HD was not associated with a significantly lower incidence of hospitalizations due to cardiovascular, cardiorespiratory or respiratory disease or any cause regardless of frailty status. Conclusions In this post-hoc analysis of a large-scale pragmatic trial, QIV-HD was similarly associated with a lower incidence of hospitalizations for pneumonia and influenza among older adults irrespective of frailty status. The potential benefit of QIV-HD versus QIV-SD may therefore be applicable regardless of frailty status. However, our results suggest that QIV-HD is associated with a lower risk of all-cause mortality among adults with low frailty only.Figure 1.