Developing a system to estimate the severity of influenza infection in England: Findings from a hospital-based surveillance system between 2010/2011 and 2014/2015

The UK Severe Influenza Surveillance System (USISS) was established following the 2009 influenza pandemic to monitor severe seasonal influenza. This article describes the severity of influenza observed in five post-2009 pandemic seasons in England. Two key measures were used to assess severity: impact measured through the cumulative incidence of laboratory-confirmed hospitalised influenza and case severity through the proportion of confirmed hospitalised cases admitted into intensive care units (ICU)/high dependency units (HDU). The impact of influenza varied by subtype and age group across the five seasons with the highest crude cumulative hospitalisation incidence for influenza A/H1N1pdm09 cases in 2010/2011 and in 0-4 year olds each season for all-subtypes. Case severity also varied by subtype and season with a higher hospitalisation: ICU ratio for A/H1N1pdm09 and older age groups (older than 45 years). The USISS system provides a tool for measuring severity of influenza each year. Such seasonal surveillance can provide robust baseline estimates to allow for rapid assessment of the severity of seasonal and emerging influenza viruses.