Outcomes, follow-up and covariates

We defined the following outcomes: (1) hospitalisation listing influenza (main and contributory diagnoses based on data from the NPR, and with influenza defined as ICD-10=J09 J11), and (2) death from influenza (main and contributory causes based on data from the Cause of Death register). Hospitalisation and deaths listing any cause were also identified to contextualise the influenza-specific outcomes. We followed individuals from the start of the influenza season to the first recorded event of interest (ie, multiple events within each influenza season were not allowed), or censoring at death, migration from Sweden or end of the influenza season.

Information on age, sex, region, socioeconomic factors (education, civil status and country of birth), influenza hospitalisation during the previous year and comorbidities (history of cancer, diabetes, heart failure, ischaemic heart disease, lung disease, stroke, surgery, venous thrombotic event and kidney failure) at the start of each influenza season was obtained from the NPR and the Prescribed Drug Register (online supplemental table S3).