Safety and immunogenicity of seven COVID-19 vaccines as a third dose (booster) following two doses of ChAdOx1 nCov-19 or BNT162b2 in the UK (COV-BOOST): a blinded, multicentre, randomised, controlled, phase 2 trial - Munro, APS; Janani, L; Cornelius, V; Aley, PK; Babbage, G; Baxter, D; Bula, M; Cathie, K; Chatterjee, K; Dodd, K; Enever, Y; Gokani, K; et al. (The Lancet, 2021)