Alliance for Pandemic Preparedness

February 23, 2021

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

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Pooled analysis of four randomized placebo-controlled trials (total n=24,422) of the Oxford-AstraZeneca ChAdOx1 nCov-19 (AZD1222) vaccine show that overall efficacy against symptomatic SARS-CoV-2 infection >14 days after the second dose was 66.7% (95%CI: 57.4-74.0%), with 84 cases (1% cumulative incidence) in the 8,597 participants in the vaccine group and 248 cases (2.9% cumulative incidence) in the 8,581 participants in the control group.

Efficacy appears to have been greater when doses were administered ≥12 weeks apart. Among those receiving two standard doses, the vaccine efficacy after the second dose was 81.3% (95%CI: 60.3-91.2%) when doses were ≥12 weeks apart and 55.1% (95%CI: 33.0-69.9%) when doses were <6 weeks apart. Exploratory analyses showed that vaccine efficacy after a single standard dose of vaccine from day 22 to day 90 after vaccination was 76.0% (59.3–85.9%)

Participants included in this pooled analysis were from studies in the UK (COV001) and (COV002), Brazil (COV003), and South Africa (COV005). [EDITORIAL NOTE: A pre-print version of this analysis was summarized in this report on February 3, 2021.]

Voysey et al. (Feb 19, 2021). 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. The Lancet. https://doi.org/10.1016/S0140-6736(21)00432-3