The ChAdOx1 nCoV-19 (AZD1222) vaccine has been approved for emergency use by the UK regulatory authority, MHRA, with a regimen of two standard doses given with an interval of between 4 and 12 weeks. The planned rollout in the UK will involve vaccinating people in high risk categories with their first dose immediately, and delivering the second dose 12 weeks later.

In this paper are provided both a further prespecified pooled analysis of trials of ChAdOx1 nCoV-19 and exploratory analyses of the impact on immunogenicity and efficacy of extending the interval between priming and booster doses. In addition, it’s shown the immunogenicity and protection afforded by the first dose, before a booster dose has been offered

The paper presents data from phase III efficacy trials of ChAdOx1 nCoV-19 in the United Kingdom and Brazil, and phase I/II clinical trials in the UK and South Africa, against symptomatic disease caused by SARS-CoV-2. The data cut-off date for these analyses was 7^th December 2020.

The accumulated cases of COVID-19 disease at this cut-off date exceeds the number required for a pre-specified final analysis, which is also presented. As previously described, individuals over 18 years of age were randomised 1:1 to receive two standard doses (SD) of ChAdOx1 nCoV-19 (5×10¹⁰ viral particles) or a control vaccine/saline placebo. In the UK trial efficacy cohort a subset of participants received a lower dose (LD, 2.2×10¹⁰ viral particles) of the ChAdOx1 nCoV-19 for the first dose. All cases with a nucleic acid amplification test (NAAT) were adjudicated for inclusion in the analysis, by a blinded independent endpoint review committee. Studies are registered at ISRCTN89951424 and ClinicalTrials.gov; NCT04324606, NCT04400838, and NCT04444674.

17,177 baseline seronegative trial participants were eligible for inclusion in the efficacy analysis, 8948 in the UK, 6753 in Brazil and 1476 in South Africa, with 619 documented NAAT +ve infections of which 332 met the primary endpoint of symptomatic infection >14 days post dose 2.

The primary analysis of overall vaccine efficacy >14 days after the second dose including LD/SD and SD/SD groups, based on the prespecified criteria was 66.7% (57.4%,  74.0%). There were no hospitalisations in the ChAdOx1 nCoV-19 group after the initial 21 day exclusion period, and 15 in the control group.Vaccine efficacy after a single standard dose of vaccine from day 22 to day 90 post vaccination was 76% (59%, 86%), and modelled analysis indicated that protection did not wane during this initial 3 month period.

Similarly, antibody levels were maintained during this period with minimal waning by day 90 day (GMR 0.66, 95% CI 0.59, 0.74).In the SD/SD group, after the second dose, efficacy was higher with a longer prime-boost interval: VE 82.4% 95%CI 62.7%, 91.7% at 12+ weeks, compared with VE 54.9%, 95%CI 32.7%, 69.7% at <6 weeks.

These observations are supported by immunogenicity data which showed binding antibody responses more than 2-fold higher after an interval of 12 or more weeks compared with and interval of less than 6 weeks GMR 2.19 (2.12, 2.26) in those who were 18-55 years of age.

ChAdOx1 nCoV-19 vaccination programmes aimed at vaccinating a large proportion of the population with a single dose, with a second dose given after a 3 month period is an effective strategy for reducing disease, and may be the optimal for rollout of a pandemic vaccine when supplies are limited in the short term.