OBJECTIVES: In 2000, Ontario initiated the world’s first universal influenza immunization program (UIIP).  Our objective was to simulate the effect of this program on influenza attack rates using a new multi-strain dynamic influenza model.  We compared our model results to a previous study of pre- and post-UIIP rates of influenza-associated events in Ontario. METHODS: Our model is age-stratified, compartmental, and explicitly tracks two A strains (H1N1, H3N2) and two B lineages (Victoria, Yamagata).  It also incorporates transmission rate seasonality, and accounts for non-homogeneous mixing among age groups via a contact matrix. U.S. age-stratified average yearly attack rates and the fraction of influenza A vs. B cases were used as calibration targets.  The resulting posterior sets of natural history parameters were run together with Ontario demographic and vaccine coverage data.  RESULTS: Our simulations showed the following post-UIIP reduction in yearly attack rate: <5yrs: 48(31-62)%; 5-19yrs: 49(33-64)%; 20-49yrs: 52(36-66)%; 50-64yrs: 58(42-71)%; 65-74yrs: 59(43-72)%; 75-84yrs: 58(41-71)%; 85+yrs: 57(40-70)%.  We compared our results to Kwong et al. (2008, PLoS Med 5(10): e211)’s analysis of post-UIIP reductions in influenza-associated events, making the assumption that within each age group, event rates scale linearly with attack rates.  Though qualitatively similar, overall their % reductions are at the upper end of our results.  CONCLUSIONS: Our model results support the conclusion that Ontario’s Universal Influenza Immunization Program has brought about a significant reduction in the province’s incidence of influenza.  Though the relative increase in vaccine uptake due to the UIIP was greatest below age 50, reductions in attack rate were greatest in ages 50 and over, indicating  indirect benefit for the elderly.  Reductions inferred by Kwong et al. (2008) are on the high side of our results, but this is consistent with their assertion that post-UIIP influenza seasons across Canada were intrinsically less severe during the study period.