Clinical Trial Info

Phase III Double-blind, Placebo-controlled Study of AZD1222 for the Prevention of COVID-19 in Adults

Authored by
Last Reviewed
September 30, 2021

The aim of the Phase 3 study is to assess the safety, efficacy, and immunogenicity of two doses of AZD1222 compacted with placebo for the prevention of severe COVID-19 15 days or more after the second dose in 32,459 adult participants.

These participants were in the United States, Chile, and Peru and including older adults.


The results of this study were published in the New England Journal of Medicine on September 29, 2021.

High vaccine efficacy was consistent across a range of demographic subgroups. In the fully vaccinated analysis subgroup, no severe or critical symptomatic Covid-19 cases were observed among the 17,662 participants in the AZD1222 group; 8 cases were noted among the 8550 participants in the placebo group (<0.1%). The estimated vaccine efficacy for preventing SARS-CoV-2 infection (nucleocapsid antibody seroconversion) was 64.3% (95% CI, 56.1 to 71.0; P<0.001). SARS-CoV-2 spike protein binding and neutralizing antibodies increased after the first dose and increased further when measured 28 days after the second dose.


AZD1222 was safe and efficacious in preventing symptomatic and severe Covid-19 across diverse populations that included older adults.