Comparative effectiveness of adjuvanted versus high-dose seasonal influenza vaccines for older adults: a systematic review and meta-analysis

Objectives: MF59-adjuvanted standard-dose and nonadjuvanted high-dose seasonal influenza vaccines have been developed to protect the elderly at high risk of severe complications. This study aimed to summarize the available evidence on the comparative efficacy/effectiveness of these two vaccines. Methods: A systematic literature review of experimental and observational studies were conducted according to the preferred reporting items for systematic reviews and meta-analyses guidelines. When possible, the extracted effect sizes were pooled in random-effects meta-analyses. Results: Ten studies were identified. Of these, no head-to-head randomized controlled trials were identified. All available studies had retrospective cohort design and large sample sizes, were conducted in the United States between the 2016-2017 and 2019-2020 seasons, and were at moderate risk of bias. Relative effectiveness estimates were limited to nonlaboratory-confirmed clinical end points, such as medical encounters including hospitalizations. Although most pooled relative effectiveness estimates were close to null, few statistically significant pooled effect sizes were small in magnitude, moved in opposite directions, and depended on the study sponsor and specificity of influenza-related outcomes. Conclusion: At present, MF59-adjuvanted standard-dose and nonadjuvanted high-dose vaccines appear to have similar effectiveness in preventing seasonal influenza in the elderly, and no conclusive recommendations on the preference of one vaccine over another could be drawn.