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

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.