Characterization and circulation of seasonal influenza viruses in Madrid, 2010-2016.

Influenza virus infection is a major health care burden and is associated with significant morbidity and mortality. The 2009 influenza pandemic highlighted the importance of influenza surveillance. The objective of this study was to assess the epidemiology and activity of influenza A and B viruses in adults and children in the post-pandemic period with a special focus on the pediatric population. We performed a retrospective descriptive study involving adults and children with influenza-like illness at the Clinico San Carlos Hospital (Madrid, Spain) over six influenza seasons, between August 2010 and April 2016. Respiratory specimens were collected from 3131 patients and routinely processed for influenza diagnosis. Epidemiological analysis was performed in terms of gender, age, and seasonal distribution. Globally, Influenza A and B viruses were detected in the respiratory specimens of 696 (22.2%) of the 3131 studied population. Among all influenza positive specimens, 142 (20.4%) were influenza A(H1N1)pdm09, 61 (8.8%) were influenza A(H3N2), 321 (46.1%) were untypeable influenza A viruses and 166 (23.9%) were influenza B. Co-infection by both influenza A and B viruses was detected in six patients (0.9%). Meanwhile, co-infection with other non-influenza respiratory viruses was identified in 5 children and 20 adults. Influenza A(H1N1)pdm09 virus activity has been significantly high since the 2009 pandemic and has gradually replaced the previously circulating seasonal influenza A(H1N1) virus. Moreover, influenza A(H3N2) virus activity remained at low levels during the last winter season while influenza B virus isolates increased significantly over the past 2 years.