AZD1222 (ChAdOx1 nCoV-19) vaccine was effective and safe to prevent symptomatic and severe coronavirus disease across diverse populations that incorporated older adults.

A study was carried out to explore the effectiveness, immunogenicity, and safety of two doses of AZD1222 versus placebo to prevent the onset of symptomatic and severe COVID-19 fifteen days or more following the 2^nd dose in adults, including older adults.

This ongoing, placebo-controlled, phase 3, randomized, double-blind trial enrolled 32,451 people. The recruited subjects underwent randomization to receive two intramuscular injections of either AZD1222 (n=21,635) or saline placebo (n=10,816) administered four weeks apart on days 1 and 29. 

AZD1222 was found to be safe, with reduced occurrences of severe and medically attended noxious events and noxious events of special interest. The occurrences were comparable to those noted in the placebo group. In both groups, solicited local and systemic reactions were usually moderate or mild. The overall estimated vaccine efficacy was 74% and estimated vaccine efficacy was 83.5% in individuals 65 years of age or older.

The high vaccine efficacy showed consistency across a range of demographic subgroups. In the fully vaccinated analysis subgroup, no serious or critical symptomatic coronavirus cases were noted among 17,662 people in the AZD1222 group. In total, 8 cases were reported among the 8550 people in the placebo group. The estimated efficacy of the vaccine to prevent coronavirus infection (nucleocapsid antibody seroconversion) was 64.3%. The SARS-CoV-2 spike protein binding and neutralizing antibodies elevated following the 1st dose and elevated further when estimated twenty-eight days following the 2nd dose.

In a diverse adult population, two doses of AZD1222 given four weeks apart were 74% efficacious overall to prevent symptomatic illness 15 days or more following the 2^nd dose.