Sample collection
Serum samples were collected between 19th March 2021 and 10th September 2021. Peripheral blood was collected in serum tubes (BD Vacutainer SS II Advanced, United Kingdom, Cat #456010) at study visits 28 (± 2) days following the first (ChAdOx1-S, BNT162b2) or second (ChAdOx1-S, BNT162b2, CoronaVac) COVID-19 vaccine dose. The recommended 21-day window between the first and second dose of CoronaVac precluded assessment of antibody responses 28 days after the first dose of CoronaVac. Peripheral blood collected in serum tubes was centrifuged and serum samples were stored at -80°C prior to testing.For plasma samples, peripheral blood was collected into lithium heparin tubes (Greiner BioOne – Austria, Cat #455084), at baseline, 3 months or 6-months after randomisation as part of the BRACE trial. Peripheral blood was centrifuged, and plasma samples were stored at -80°C. The participant’s most recent plasma sample available prior to COVID-19 vaccination was used in this study.