Covariates

We included possible confounders suspected to be associated with severity of COVID-19 or epidemiologically associated with both SARS-CoV-2 infection and hospitalisation, thus forward denoted basic covariates. These basic covariates included sex, age at sampling, time period (week of the sampling according to ISO 8601 standard, starting Monday; week 53 is a leap week), geographical region of sampling (Capital, Central Denmark, North Denmark, Zealand, Southern Denmark, or missing region name), and comorbidities (diabetes, adiposity, cancer, neurological diseases, nephrological diseases, haematological diseases, cardiac diseases, respiratory disorders, immunological diseases, and other comorbid diseases based on the preceding 5 years of hospital admission diagnoses).²⁴ Additional covariates were included later in the study and only for sensitivity analyses or to explore effects of B.1.1.7 on admission risk in relevant subgroups; the additional covariates included test track, which was also subdivided by Ct value lower than 30 and equal or higher than 30 in the community track (individuals with missing Ct values were excluded when using this information); ethnicity (second generation [both parents born abroad], Danish-born, or born abroad);²⁵ comorbidity based on the Register of Selected Chronic Diseases and Severe Mental Disorders (asthma, dementia, type 1 diabetes, type 2 diabetes, chronic obstructive pulmonary disease, rheumatoid arthritis, osteoporosis, schizophrenia),²⁶ living in a long-term care facility [LTCF] for older people on the basis of the address database of LTCFs for older people (the majority being older than 65 years) and the address at sample date;^{27, 28} being a health-care worker on the basis of work authorisations in the Danish Register of Healthcare Professionals;²⁹ and SARS-CoV-2 vaccination status on the basis of the Danish Vaccination Register.³⁰ Information on intensive care unit treatment was obtained from the National Patient Register.²⁴