Transmissibility of influenza during the 21st-century epidemics, Spain, influenza seasons 2001/02 to 2017/18

Abstract

Background: Understanding influenza seasonality is necessary for determining policies for influenza con- trol. Aim: We characterised transmissibility during seasonal influenza epidemics, including one influenza pandemic, in Spain during the 21th century by using the moving epidemic method (MEM) to calculate intensity levels and estimate differences across seasons and age groups. Methods: We applied the MEM to Spanish Influenza Sentinel Surveillance System data from influ- enza seasons 2001/02 to 2017/18. A modified version of Goldstein’s proxy was used as an epidemiological- virological parameter. We calculated the average start- ing week and peak, the length of the epidemic period and the length from the starting week to the peak of the epidemic, by age group and according to seasonal virus circulation. Results: Individuals under 15 years of age presented higher transmissibility, especially in the 2009 influenza A(H1N1) pandemic. Seasons with dominance/co-dominance of influenza A(H3N2) virus presented high intensities in older adults. The 2004/05 influenza season showed the highest influ- enza-intensity level for all age groups. In 12 seasons, the epidemic started between week 50 and week 3. Epidemics started earlier in individuals under 15 years of age (−1.8 weeks; 95% confidence interval (CI):−2.8 to −0.7) than in those over64 years when influenza B virus circulated as dominant/co-dominant. The aver- age time from start to peak was 4.3 weeks (95% CI: 3.6–5.0) and the average epidemic length was 8.7 weeks (95% CI: 7.9–9.6). Conclusions: These findings provide evidence for intensity differences across sea- sons and age groups, and can be used guide public health actions to diminish influenza-related morbidity and mortality.