Seasonal dynamics of influenza viruses and age distribution of infected individuals across nine seasons covering 2009–2018 in Taiwan

Background/Purpose A swine-origin influenza A/H1N1 virus (termed A/H1N1pdm) caused a pandemic in 2009 and has continuously circulated in the human population. To investigate its possible ecological effects on circulating influenza strains, the seasonal patterns of influenza viruses and the respective age distribution of infected patients were studies. Methods The data obtained from national influenza surveillance systems in Taiwan from July 2009 to June 2018 were analyzed. Results The A/H1N1pdm and A/H3N2 strains usually caused a higher ratio of severe to mild cases than influenza B. New variants of A/H1N1pdm and A/H3N2 emerged accompanied by a large epidemic peak. However, the new influenza B variants intended to circulate for several seasons before causing a large epidemic. The major group of outpatients affected by A/H1N1pdm were aged 13–23 years in the pandemic wave, and the age range of infected individuals gradually shifted to 24–49 and 0–6 years across seasons; A/H1N1pdm-infected inpatients were aged 24–49 years in 2009–2011, and the age range gradually switched to older groups aged 50–65 and >65 years. Individuals aged 0–6 or 24–49 years accounted for the majority of A/H3N2-infected outpatients across seasons, whereas most of the inpatients affected by A/H3N2 were aged >65 years. Conclusion Understanding the effects of new variants and changes in dominant circulating viral strains on the age distribution of the affected human population, disease severity and epidemic levels is useful for the establishment of fine-tuned strategies for further improvement of influenza control.