Influenza hospitalization burden by subtype, age, comorbidity and vaccination status: 2012/13 to 2018/19 seasons, Quebec, Canada

BackgroundThe primary objective of influenza immunization programs is to reduce the risk and burden of severe outcomes. To inform optimal program strategies, we monitored influenza hospitalizations over several seasons of varying subtype predominance, stratified by age, comorbidity and vaccination status. MethodsWe assembled data from an active hospital-based surveillance network involving systematic swabbing and PCR-confirmation of influenza virus infection by type/subtype during peak-weeks of seven influenza seasons (2012/13 to 2018/19) in Quebec, Canada. We estimated seasonal, population-based incidence of influenza-associated hospitalizations (interpreted as risk) by subtype, age, comorbidity and vaccine status, and derived the number-needed-to-vaccinate to prevent one hospitalization per stratum. ResultsThe average seasonal incidence of influenza-associated hospitalization was 89/100,000 (95%CI: 86, 93), lower during A(H1N1) (49-82/100,000) than A(H3N2) seasons (73-143/100,000). Overall risk followed a J-shaped age pattern, highest among infants 0-5 months and adults [&ge;]75 years. Hospitalization risks were highest for children <5 years during A(H1N1) but for adults [&ge;]75 years during A(H3N2) subtype- predominant seasons. Age-adjusted hospitalization risks were 7-fold higher among individuals with versus without comorbidities (214 versus 30/100,000). The number-needed-to-vaccinate to prevent hospitalization was 82-fold lower for [&ge;]75-years-olds with comorbidity (n=1,995), who comprised 39% of all hospitalizations, than for healthy 18-64-year-olds (n=163,488), who comprised just 6% of all hospitalizations. ConclusionsIn the context of broad-based influenza immunization programs (targeted or universal), severe outcome risks should be simultaneously examined by subtype, age, comorbidity, and vaccine status. Policymakers require such detail to prioritize further promotional efforts and expenditures toward the greatest and most efficient program impact. 40-word summaryThis hospital-based study involving systematic PCR testing over seven seasons revealed important differences in influenza hospitalization risk by subtype, age, comorbidity, and vaccination status. The findings highlight the need for data-driven decision-making to optimize vaccination strategies and minimize healthcare burden.