Time-sensitive testing pressures and COVID-19 outcomes: are socioeconomic inequalities over the first year of the pandemic explained by selection bias?

BackgroundThere are many ways in which selection bias might impact COVID-19 research. Here we focus on selection for receiving a polymerase-chain-reaction (PCR) SARS-CoV-2 test and how known changes to selection pressures over time may bias research into COVID-19 infection.MethodsUsing UK Biobank (N = 420,231; 55% female; mean age = 66.8 [SD = 8 & BULL;11]) we estimate the association between socio-economic position (SEP) and (i) being tested for SARS-CoV-2 infection versus not being tested (ii) testing positive for SARS-CoV-2 infection versus testing negative and (iii) testing negative for SARS-CoV-2 infection versus not being tested. We construct four distinct time-periods between March 2020 and March 2021, representing distinct periods of testing pressures and lockdown restrictions and specify both time-stratified and combined models for each outcome. We explore potential selection bias by examining associations with positive and negative control exposures.ResultsThe association between more disadvantaged SEP and receiving a SARS-CoV-2 test attenuated over time. Compared to individuals with a degree, individuals whose highest educational qualification was a GCSE or equivalent had an OR of 1 & BULL;27 (95% CI: 1 & BULL;18 to 1 & BULL;37) in March-May 2020 and 1 & BULL;13 (95% CI: 1.& BULL;10 to 1 & BULL;16) in January-March 2021. The magnitude of the association between educational attainment and testing positive for SARS-CoV-2 infection increased over the same period. For the equivalent comparison, the OR for testing positive increased from 1 & BULL;25 (95% CI: 1 & BULL;04 to 1 & BULL;47), to 1 & BULL;69 (95% CI: 1 & BULL;55 to 1 & BULL;83). We found little evidence of an association between control exposures, and any considered outcome.ConclusionsThe association between SEP and SARS-CoV-2 testing changed over time, highlighting the potential of time-specific selection pressures to bias analyses of COVID-19. Positive and negative control analyses suggest that changes in the association between SEP and SARS-CoV-2 infection over time likely reflect true increases in socioeconomic inequalities.

Automated summary

This study highlights the potential impact of selection bias on COVID-19 research, specifically in relation to the selection for receiving a SARS-CoV-2 test. The findings suggest that the association between socioeconomic position (SEP) and testing for SARS-CoV-2 infection changed over time, with the magnitude of the association attenuating or increasing.