Delaying the second dose of covid-19 vaccines

On 30 December 2020, the UK announced a deviation from the recommended protocol for the Pfizer-BioNTech covid-19 vaccine, prolonging the interval between doses from 3 to 12 weeks. 1 2 Similar decisions were made for the Oxford-AstraZeneca vaccine, for which a longer gap between doses had been shown to improve efficacy in some age groups. 3 The stated intention was to maximise benefit with limited supplies and to minimise hospital admissions and deaths. For the Pfizer-BioNTech vaccine, the decision to delay the second dose was based on extrapolations from phase III trial data showing an efficacy of 89% 15-21 days after the first dose. 4 5 At the time, Pfizer did not support the decision, stating that high efficacy could not be guaranteed.