Post-COVID-19 Burden of Severe Influenza in England Among Overall Population and High-Risk Groups: Results From the INFORM Study, 2022–2023

Abstract Introduction We describe the burden of severe influenza in England overall, and in risk groups (as defined by age, immunocompromise, and other comorbidities) and those with recent COVID-19. Methods The INFORM (INvestigation oF cOvid-19 Risk among iMmunocompromised populations) cohort uses pseudonymised, routinely collected electronic health records of individuals aged ≥ 12 years from a random 25% sample of England’s population (N = 12.1 million). In this analysis, during 1 July 2022 to 30 June 2023, crude incidence rates (IRs) and age- and sex-adjusted IR ratios were estimated for severe influenza outcomes of hospitalisation, intensive care unit (ICU) admission and death. The analyses were repeated for outcomes of severe influenza with recent COVID-19 (< 6 months). Results The analysis sample comprised 12,075,920 individuals aged ≥ 12 years; 3.9% were immunocompromised. Overall, IRs of severe influenza were 10.51 (hospitalisations), 0.35 (ICU admissions) and 0.73 (deaths) per 10,000 person-years. Of those with influenza hospitalisation, 13.1% had recent COVID-19. Overall, influenza hospitalisation IRs per 10,000 person-years increased with advancing age, from 3.56 for adolescents aged 12–17 years to 58.43 for adults aged ≥ 80 years. Compared with other age groups, among hospitalised individuals, the highest intensity of management occurred in adolescents aged 12–17 years, based on the proportion of influenza hospitalisations that involved ICU admission (6.2%) or mechanical ventilation (3.0%) in this age group. Increased age- and sex-adjusted risk of severe influenza outcomes was observed among individuals with immunocompromise and other comorbidities. Conclusions Seasonal severe influenza was observed in the general population aged ≥ 12 years in England in 2022–2023, with a disproportionately higher burden recorded in risk groups (as defined by age, immunocompromise, recent COVID-19 and other comorbidities). These data reinforce continued efforts to immunise individuals against seasonal influenza. Trial Registration International Standard Randomised Controlled Trial Number Registry: ISRCTN53375662. Graphical Abstract