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UPDATE: AstraZenca-Oxford Vaccine 79% Effective in US Phase 3 Interim Analysis


Updated: The vaccine, AZD1222, showed 100% efficacy against severe disease/hospitalization and, notably, 80% efficacy in trial participants aged ≥65 years, the company reports.

UPDATE: US health authorities expressed concern early Tuesday morning, March 23, 2021, that study results detailed below are based on outdated data. The National Institute of Health has urged AstraZeneca to work with the Data and Safety Monitoring Board to review the data and make sure the data released is the most current.

Results of a phase 3 US safety and efficacy analysis reported today showed that the COIVD-19 vaccine developed by AstraZeneca is 79% effective against symptomatic illness and 100% effective against severe infection and hospitalization, according to a company press release.

The company reports that efficacy of AZD1222 was consistent across age and ethnicity and, importantly, that efficacy in participants aged greater than or equal to 65 years was 80%.

This interim analysis of ADZ1222, developed in partnership with Oxford University, was based on 32 449 participants from the US, Chile, and Peru.

In light of reports from several European countries of blood clots associated with the vaccine and temporary suspension of vaccine programs, the independent data safety monitoring board for the study consulted with an independent neurologist to conduct a specific review of thrombotic events as well as cerebral venous thrombosis. There was no increased risk of thrombosis or related events among the 21 583 participants receiving at least one dose.


~79% white/Caucasian
~8% black/African American
~4% native American
~4% Asian
~22% Hispanic

Approximately 20% of participants were age ≥65 years, and approximately 60% had co-morbidities associated with an increased risk for progression of severe COVID-19, such as diabetes, severe obesity, or cardiac disease.

“These findings reconfirm previous results observed in AZD1222 trials across all adult populations but it’s exciting to see similar efficacy results in people over 65 for the first time. This analysis validates the AstraZeneca COVID-19 vaccine as a much-needed additional vaccination option, offering confidence that adults of all ages can benefit from protection against the virus," said Ann Falsey, professor of medicine, University of Rochester School of Medicine, US, and co-lead principal investigator for the trial, in the AstraZeneca statement.

Participants in the US phase 3 trial received 2 vaccines doses, 4 weeks apart. The company had previously reported evidence that an interval of up to 12 weeks between doses yielded greater efficacy, suggesting that the longer period could both increase efficacy and accelerate the number of people who can receive a first dose.

AstraZeneca said it will continue to analyze the data and prepare for the primary analysis to be submitted to the US Food and Drug Administration for emergency use authorization in the coming weeks.

The U.S. government placed an order for 300 million doses of the vaccine last year, at a cost of $1.2 billion.

For more COVID-19 coverage for primary care, visit our COVID-19 Resource Page.

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