Combined Oxaliplatin with 5-Fluorouracil for Effective Chemotherapy Against Gastric Cancer in Animal Model.

BACKGROUND

Seasonal influenza vaccination programs aim to provide protection before the virus circulation begins. However, vaccine effectiveness (VE) may decline within a single influenza season due to waning immunity, antigenic drift, and season-specific factors, raising questions about optimal vaccination timing. The purpose of this review was to synthesize evidence on intraseasonal waning of seasonal influenza VE and examine the implications and key considerations for optimizing the timing of seasonal vaccination in Canada.

METHODS

A scoping review was conducted, with searches of MEDLINE, Embase, Scopus, Cochrane Library, and ProQuest Public Health Database that identified studies published from 2010 to June 2024, with an update in July 2025. Eligible studies included clinical trials, observational studies, systematic reviews, and modelling studies reporting at least two VE estimates by time since vaccination within a season or assessing vaccination timing and its impact on influenza outcomes.

RESULTS

Forty-nine studies met inclusion criteria, including 37 assessing intraseasonal waning and 12 examining vaccination timing. Overall, VE was generally highest within one to three months post-vaccination and declined over the season (e.g., three to six months post-vaccination). Waning was more consistently observed for influenza A, particularly A(H3N2), and among adults aged 60 years and older. Modelling studies suggested that delaying vaccination could reduce influenza burden under select late-peaking or fast-waning scenarios; however, estimated benefits were generally small and highly sensitive to assumptions.

CONCLUSION

Intraseasonal waning of influenza VE is consistently observed, particularly for influenza A and in older adults, but represents a gradual decline that does not typically undermine season-long effectiveness. Evidence does not support substantial or consistent population-level benefits from delaying vaccination.