Covariates
Age: Categorised as 18-39 years, 40-49 years, 50-59 years, 60-69 years, 70-79 years and ≥ 80 years for exploratory analysis (in line with previous reports)13 and treated as continuous in the final multivariate model.
Sex: Male or female.
Ethnicity: Coded in categories used by NHS Digital: White, South Asian (Bangladeshi, Indian, Pakistani), Other Asian, Black, Mixed, Other, not stated.
Deprivation: Recorded using the Index of Multiple Deprivation (IMD) for the Lower Super Output Area (LSOA) of the patients’ home address, with scores categorised into quintiles based on national averages.
Comorbidities: These were the 14 comorbidities used to construct the Charlson Comorbidity Index (Table 1) .14 The comorbidity was deemed present if it was recorded in HES as a secondary diagnosis in the index admission or as a primary or secondary diagnosis in any admission during the previous year, in accordance with the recommendations of Quan et al .15
Obesity: Recorded as present if the ICD-10 code E66 was used as a diagnostic code during the admission.
Discharge date: Categorised into day or month of discharge (starting from 1st March) depending on the analysis undertaken. Monthly data were used for descriptive statistics. Daily data were used for the final multivariable model.
Admission date: Categorised into month of admission. Only admissions from 1st February to 31st August 2020 were used, to avoid biasing the data at the end of the study period due to many patients admitted in September and October still being in hospital on 31st October.