Implementation and adherence to regular asymptomatic testing in a COVID-19 vaccine trial

For pathogens which cause infections that present asymptomatically, evaluating vaccine efficacy (VE) against asymptomatic infection is important for understanding a vaccine's potential epidemiological impact. Regular testing for subclinical infections is a potentially valuable strategy but its success hinges on participant adherence and minimising false positives. This paper describes the implementation and adherence to weekly testing in a COVID-19 vaccine trial. COV002 was a phase 2/3 trial assessing the ChAdOx1 nCoV-19 vaccine against SARS-CoV-2. Asymptomatic infections were detected using weekly self-administered swabs for RT-PCR testing. We analysed adherence using mixed-effects regression models and estimated the probability of true and false positive asymptomatic infections using estimates of adherence and testing characteristics. 356,551 tests were self-administered by 10,811 participants during the 13-month follow-up. Median adherence was 75.0% (IQR 42·6–90·9), which translated to a 74·5% (IQR 50·9–78·8) probability of detecting a positive asymptomatic infection during the swabbing period, and between 21 and 96 false positives during VE evaluation. The odds of returning a swab declined by 8% per week and further after testing positive and unblinding. Adherence was higher in older age groups, females and non-healthcare workers. The COV002 trial demonstrated the feasibility of running a long-term regular asymptomatic testing strategy. This information could be valuable for designing future phase III vaccine trials in which infection is an outcome. UK Research and Innovation, National Institutes for Health Research (NIHR), Coalition for Epidemic Preparedness Innovations, NIHR Oxford Biomedical Research Centre, Thames Valley and South Midland's NIHR Clinical Research Network, AstraZeneca.