Background: Frailty is a risk factor for adverse influenza-related outcomes. We assessed the effectiveness of high-dose (HD-IIV) versus standard-dose (SD-IIV) influenza vaccination according to frailty score (FS).
Methods: This was a post-hoc analysis of the randomized feasibility trial of HD-IIV versus SD-IIV conducted during the 2021-2022 influenza season in older adults aged 65-79 years. We assessed prespecified outcomes including hospitalizations and mortality as time to first and recurrent events. Frailty was defined according to the Hospital Frailty Risk Score.
Results: Among 12,477 included participants (mean age 71.7 years, 47.1% female), 10,689 (85.7%) were categorized as having low frailty (<5 points) and 1,784 (14.3%) had intermediate or high frailty (≥5 points). HD-IIV versus SD-IIV was associated with a lower risk of first and recurrent hospitalizations for pneumonia or influenza regardless of FS (Low frailty: 22 events, HR 0.37, 95% CI 0.15- 0.96; 25 recurrent events, IRR 0.31, 95% CI 0.11- 0.84. Intermediate or high frailty: 16 events, HR 0.33, 95% CI 0.11-1.01; 18 recurrent events, IRR 0.28, 95% CI 0.09-0.92. Pinteraction 0.92 and 0.93, respectively). FS modified the association of HD-IIV versus SD-IIV with all-cause mortality (Pinteraction 0.022), with an association with reduced risk in participants with low frailty only (43 events, HR 0.26, 95% CI 0.13 to 0.55).
Conclusion: HD-IIV was associated with a lower risk of first and recurrent hospitalizations for pneumonia and influenza compared with SD-IIV and may be preferred for older adults irrespective of frailty status. FS modified the association of HD-IIV versus SD-IIV with all-cause mortality.
Keywords: Influenza; frailty; hospitalizations; mortality; pragmatic; randomized controlled trial; vaccine.
© The Author(s) 2025. Published by Oxford University Press on behalf of Infectious Diseases Society of America.