Adjuvanted Influenza Vaccines Versus Other Influenza Vaccines in Adults Aged ≥60 Years: A Systematic Review and Meta-Analysis of Relative Effectiveness for Preventing Influenza-Related Hospitalization
Abstract
Background/Objectives: Seasonal influenza vaccines have been developed to reduce the risk of severe complications in older adults, who are at increased risk of influenza-related morbidity and mortality. This study aimed to synthesize the available evidence on the comparative effectiveness of MF59-adjuvanted versus high-dose and non-adjuvanted influenza vaccines in preventing hospitalization/ED access. Methods: A systematic literature review of observational and pragmatic studies was conducted in accordance with the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Eligibility was set at ≥60 years. Results: Initially, 1556 records were identified for screening by title and abstract. After removing duplicates (n = 378), 1122 were judged ineligible; 56 records were screened for full-text evaluation. The reconciled dataset contained 13 publications for ADJ versus HD (12 with overall-population estimates and one with risk-stratum estimates) and eight for ADJ versus SD; two publications contributed to both comparisons. All included studies concerned adults aged ≥65 years. In the exploratory overall displays, including explicitly identified broader outcomes, exploratory summaries across adjusted ratio measures were 0.977 (95% normal-theory CI 0.946 to 1.010) for MF59-adjuvanted versus high-dose vaccines (12 publications, 12 estimates; I2 = 75.8%; 95% prediction interval 0.891 to 1.072) and 0.818 (95% normal-theory CI 0.783 to 0.855) versus standard-dose vaccines (eight publications, eight estimates; I2 = 41.6%; 95% prediction interval 0.758 to 0.883; Hartung–Knapp CI 0.766 to 0.875). Conclusions: The exploratory synthesis did not show a clear difference between MF59-adjuvanted and high-dose vaccines. The average favored adjuvanted over standard-dose vaccines, but sensitivity to outcome selection, heterogeneity estimation and potentially overlapping populations limits interpretation.