Estimates of the risk of hospital admission for coronavirus (COVID-19) and death involving COVID-19 by vaccination status, overall and by age group, using anonymised linked data from Census 2021. Experimental Statistics.
Outcome definitions
For this analysis, we define a death as involving COVID-19 if either of the ICD-10 codes U07.1 (COVID-19, virus identified) or U07.2 (COVID-19, virus not identified) is mentioned on the death certificate. Information on cause of death coding is available in the User Guide to Mortality Statistics. We use date of occurrance rather than date of registration to give the date of the death.
We define COVID-109 hospitalisation as an inpatient episode in Hospital Episode Statistics where the primary diagnosis was COVID-19, identified by the ICD-19 codes (COVID-19, virus identified) or U07.2 (COVID-19, virus not identified). Where an individual had experienced more than one COVID-19 hospitalisation, the earliest that occurred within the study period was used. We define the date of COVID-19 hospitalisation as the start of the hospital episode.
ICD-10 code
U07.1 :
COVID-19, virus identified
U07.2:
COVID-19, virus not identified
Vaccination status is defined by the dose and the time since the last dose received
Unvaccinated:
no vaccination to less than 21 days post first dose
First dose 21 days to 3 months:
more than or equal to 21 days post second dose to earliest of less than 91 days post first dose or less than 21 days post second dose
First dose 3+ months:
more than or equal to 91 days post first dose to less than 21 days post second dose
Second dose 21 days to 3 months:
more than or equal to 21 days post second dose to earliest of less than 91 days post second dose or less than 21 days post third dose
Second dose 3-6 months:
more than or equal to 91 days post second dose to earliest of less than 182 days post second dose or less than 21 days post third dose
Second dose 6+ months:
more than or equal to 182 days post second dose to less than 21 days post third dose
Third dose 21 days to 3 months:
more than or equal to 21 days post third dose to less than 91 days post third dose
Third dose 3+ months:
more than or equal to 91 days post third dose
Model adjustments
Three sets of model adjustments were used
Age adjusted:
age (as a natural spline)
Age, socio-demographics adjusted:
age (as a natural spline), plus socio-demographic characteristics (sex, region, ethnicity, religion, IMD decile, NSSEC category, highest qualification, English language proficiency, key worker status)
Fully adjusted:
age (as a natural spline), plus socio-demographic characteristics (sex, region, ethnicity, religion, IMD decile, NSSEC category, highest qualification, English language proficiency, key worker status), plus health-related characteristics (disability, self-reported health, care home residency, number of QCovid comorbidities (grouped), BMI category, frailty flag and hospitalisation within the last 21 days.
Age
Age in years is defined on the Census day 2021 (21 March 2021). Age is included in the model as a natural spline with boundary knots at the 10th and 90th centiles and internal knots at the 25th, 50th and 75th centiles. The positions of the knots are calculated separately for the overall model and for each age group for the stratified model.
