Estimating the burden of seasonal influenza in Spain from surveillance of mild and severe influenza disease, 2010‐2016

BACKGROUND: Estimating the national burden of influenza disease is challenging. We aimed to estimate the disease burden of seasonal influenza in Spain, at the primary care and hospital level, over the 6 influenza seasons after 2009 pandemic. METHODS: We used data from the Spanish Influenza Sentinel Surveillance System to estimate weekly influenza rates and the number of influenza‐like illness (ILI) and mild confirmed influenza cases (MCIC). From the surveillance of severe hospitalized confirmed influenza cases (SHCIC), we obtained hospitalization rates and total number of SHCIC, intensive care unit (ICU) admissions and deaths in influenza hospitalized patients. We estimated both mild and severe influenza cases, overall, and by age‐group (<5, 5‐14, 15‐64, and ≥65 years). RESULTS: The highest cumulative rates of MCIC were observed in <15 years (1395‐3155 cases/100 000 population in 5‐14 years) and the lowest in ≥65 years (141‐608 cases/100 000 population). SHCIC rates revealed a characteristic U‐shaped distribution, with annual average hospitalization rates of 16.5 and 18.9 SHCIC/100, 000 p in 0‐4 years, and ≥65 years, respectively. We estimated an annual average of 866 868 cases of ILI attended in primary care (55% were MCIC), 3616 SHCIC, 1232 ICU admissions, and 437 deaths in SHCIC. The percentage of ICU admission among SHCIC was highest at 15‐64 years (42%), while the hospitalization fatality rate ranged from 1% in 0‐4 years to 18% in ≥65 years. CONCLUSIONS: The ongoing Spanish Influenza Surveillance System allowed obtaining crucial information regarding the impact of mild and severe influenza in Spain.