Covariates
For both venous and DBS results, the time a sample was kept at room temperature was calculated as the time between sample collection by the nurse/participant and receipt at MRC Epidemiology Unit. All venous blood samples were processed before COVID-19 related laboratory closure. A variable was included to describe whether DBS samples were measured before or after the COVID-19-related lab closure. Other indicators of the quality of the DBS sample considered were the visual inspection score described above and the number of blood spots on the DBS card.Responses to questionnaires were used to record age, sex, highest level of education (obtaining a degree or higher 