OBJECTIVES: Current trivalent influenza vaccines (TIVs) contain only one of the two currently circulating influenza B lineages (Victoria and Yamagata). Worldwide, in about half of all influenza seasons since co-circulation of the two B lineages commenced, the dominant B lineage did not match the one chosen for inclusion in the TIV.  Quadrivalent influenza vaccines (QIVs) would help address this problem by including both B lineages each season.  We modeled the impact of a country-wide switch from TIV to QIV on the yearly population-wide rates of influenza cases and influenza-associated events, in both Canada and the UK. METHODS: We calculated projections using a dynamic transmission model which incorporates four interacting influenza strains, transmission-rate seasonality and age-specific mixing within the population, run over a 40-year time horizon.  Influenza vaccine coverage rates in Canada and the UK were taken from public sources, TIV efficacy was obtained from a meta-analysis (Tricco et al., in press), and QIV efficacy was assumed to be similar to TIV without B lineage mismatch.  RESULTS: Across Canada, the model estimates that a switch from TIV to QIV would, in an average influenza season, avert 9% (relative) of influenza cases (=237,000 cases), 9% (=86,000) of general practitioner (GP) visits, 9% (=12,000) of emergency room (ER) visits, 8% (=2,500) of hospitalizations, and 7% (=330) of deaths.  Across the UK, the model estimates that 0.7% (=70,000) of influenza cases, 0.8% (=19,000) of GP visits, 0.8% (=600) of ER visits, 0.8% (=800) of hospitalizations, and 3% (=270) of deaths would be averted. CONCLUSIONS: In both Canada and the UK, a country-wide TIV-QIV switch is predicted to bring about a clear reduction in the burden of influenza.  The relatively greater impact of the switch in Canada is due principally to that country’s higher vaccine uptake among people younger than 65 years.