AstraZeneca announced high-level results from the US phase 3 trial of its COVID-19 vaccine, following a statement issued by the National Institute of Allergy and Infectious Diseases regarding concerns over outdated data.

The primary analysis was found to be consistent with interim data that had been released earlier this week. Among 32,449 participants, an additional 49 cases were reported, bringing the total to 190 symptomatic cases of COVID-19. Vaccine efficacy at preventing symptomatic disease 15 days or more after receiving 2 doses given 4 weeks apart was determined to be 76% (95% CI, 68-82) in the overall study population and 85% (95% CI, 58-95) in adults 65 years of age and older.

Additionally, the vaccine was found to be 100% effective at preventing severe disease and hospitalization.  In the primary analysis, there were 8 cases of severe COVID-19, all of which occurred in the placebo group.

The updated results were presented to the independent Data Safety Monitoring Board. An additional 14 possible or probable cases are still being reviewed and may change the estimates slightly, according to the Company. No safety concerns related to the vaccine were identified in the primary analysis.

AstraZeneca’s COVID-19 vaccine, AZD1222, utilizes a replication-deficient chimpanzee adenovirus to deliver a SARS-CoV-2 protein to induce a protective immune response. The phase 3, double-blind, placebo-controlled study (D8110C00001; NCT04516746) assessed the safety, efficacy, and immunogenicity of AZD1222 in individuals 18 years of age and older; approximately 20% of the participants were 65 years and older and roughly 60% had comorbidities. Study participants were randomly assigned 2:1 to receive 2 intramuscular doses of AZD1222 or saline placebo 4 weeks apart.

The new findings will be submitted to the Food and Drug Administration (FDA) in an effort to gain Emergency Use Authorization for the vaccine in the US.  If authorized, the Company plans to roll out millions of doses across the country. The AstraZeneca COVID-19 vaccine can be stored at normal refrigerated temperatures for at least 6 months.


1.      AZD1222 US phase III primary analysis confirms safety and efficacy. [press release]. AstraZeneca PLC; March 25, 2021.

2.      NIAID statement on AstraZeneca vaccine. [press release]. National Institute of Allergy and Infectious Diseases: Bethesda, Maryland; March 23, 2021.