What is the AZD-1222 coronavirus disease 2019 (COVID-19) vaccine?

Updated: Sep 24, 2021
  • Author: David J Cennimo, MD, FAAP, FACP, FIDSA, AAHIVS; Chief Editor: John L Brusch, MD, FACP  more...
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  • Pending EUA submission in the United States; approved for use in the United Kingdom and other countries 
  • Phase 3 trials in US completed March 2021
  • Phase 3 trial in UK planned to assess safety and immune response in children and young adults aged 6-17 years 

AZD-1222 (ChAdOx1 nCoV-19; AstraZeneca) is a replication-deficient chimpanzee adenoviral vector vaccine containing the surface glycoprotein antigen (spike protein) gene. It is administered as a 2-dose series 28 days apart. This vaccine primes the immune system by eliciting antibodies to attack the SARS-CoV-2 virus if it later infects the body. Owing to the testing of a different coronavirus vaccine in 2019, development for AZD-1222 was faster than that of other viral vector vaccines. 

Results of an interim analysis of the phase 3 clinical trial in the United Kingdom, Brazil, and South Africa are as follows: 

One dosing regimen (n = 2741) showed vaccine efficacy of 90% when given as a half dose, followed by a full dose at least 1 month later. Another dosing regimen (n = 8895) showed 62.1% efficacy when given as 2 full doses at least 1 month apart. The combined analysis from both dosing regimens (N = 11,636) resulted in an average efficacy of 70.4% (p < .0001 for all). From 21 days after the first dose, 10 patients in the control arm were hospitalized for COVID-19 (2 severe, 1 death). [35]  Concerns about the clinical trial implementation and data analysis have emerged because the half-dose regimen was not in the approved study design. [36] These concerns will be addressed by regulatory agencies and await publication of the trial data.

In another trial, researchers studied the second vaccine dose administered at 3 months after the first dose instead of at 1 month. There were no hospitalizations in the vaccine group after the initial 21-day exclusion period compared with 15 in the control group. Vaccine efficacy after a single standard dose of vaccine from day 22 to day 90 was 76%. Modelled analysis indicated that protection did not wane during the initial 3-month period. Similarly, antibody levels were maintained during this period with minimal waning by day 90. In the group that received 2 doses of the standard dose, vaccine efficacy was higher with a longer prime-boost interval (3 months) compared with an interval less than 6 weeks (82.4% vs 54.9% respectively). [37]

Additionally, nasal swabs were obtained from trial participants every week in the UK study, regardless of symptoms. This allowed assessment of the overall impact of the vaccine on risk of infection, and thus a surrogate for potential onward transmission. A single standard dose of AZD-1222 reduced PCR positivity by 67%, and after the second dose, reduced PCR positivity by 49.5% overall. These data indicate that ChAdOx1 nCoV-19, used in the authorized schedules, may have a substantial impact on transmission by reducing the number of infected individuals in the population. [37]  

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