Alliance for Pandemic Preparedness

February 3, 2021

Single Dose Administration , and the Influence of the Timing of the Booster Dose on Immunogenicity and Efficacy of ChAdOx1 NCoV-19 ( AZD1222 ) Vaccine

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[Pre-print, not peer-reviewed] Exploratory analysis of interim data from the University of Oxford studies of the ChAdOx1 (Oxford-AstraZeneca) vaccine suggested that lengthening the interval between vaccination doses was associated with increases in clinical efficacy. In the standard dose group (since approved by the MHRA and other international regulators), the efficacy after the second dose was 82% at 12+ weeks, compared with 55% at <6 weeks and antibody responses were more than twice as high after 12+ weeks compared to <6 weeks among those who were 18-55 years of age. Due to a mishap with calculating the concentration of study product, a subset of participants had received a lower dose (LD) of the vaccine for the first dose (approximately one half of the intended dose) which did not affect clinical efficacy. Additionally, in a subset of participants who elected not to receive the second dose, the efficacy of a sole dose of the ChAdOx1 nCoV-19 (Oxford-AstraZeneca) was 76% and protection did not decline during the 3-month period following the initial vaccination.

Participants in the study also received weekly swabs to look for detectable nucleic acid in the absence of symptoms. Overall reduction in PCR+ samples was 54% (45%-62%), suggesting a potential for reduced transmissibility.

Voysey et al. (Feb 1, 2021). Single Dose Administration , and the Influence of the Timing of the Booster Dose on Immunogenicity and Efficacy of ChAdOx1 NCoV-19 ( AZD1222 ) Vaccine. SSRN. https://ssrn.com/abstract=3777268