Vaccines to prevent COVID-19 infection are crucial for an effective global pandemic response. In The Lancet, Merryn Voysey and colleagues1Voysey M Costa Clemens SA Madhi SA et al.Single dose administration and the influence of the timing of the booster dose on immunogenicity and efficacy of ChAdOx1 nCoV-19 (AZD1222) vaccine: a pooled analysis of four randomised trials.