Vaccine Effectiveness of non-adjuvanted and adjuvanted trivalent inactivated influenza vaccines in the prevention of influenza-related hospitalization in older adults: A pooled analysis from the Serious Outcomes Surveillance (SOS) Network of the Canadian Immunization Research Network (CIRN)

•Age and frailty may impact Vaccine Effectiveness (VE), which is generally lower in older adults.•Enhanced influenza vaccines aim to optimize dysregulated immune responses in older adults.•We studied relative VE of adjuvanted vs. non-adjuvanted standard-dose influenza vaccine.•Adjusting for age and frailty, relative VE was 25%, favouring adjuvanted influenza vaccine.•Frailty has a meaningful effect on VE and should be considered in future studies.