AstraZeneca-Oxford COVID-19 vaccine 79 percent effective in US trial

Elvira Manzano
28 Mar 2021

The COVID-19 vaccine AZD1222 by AstraZeneca and the University of Oxford was 79 percent effective overall in preventing symptomatic infections – higher than observed in previous clinical trials of the vaccine – at the interim analysis of a US phase III trial.

The vaccine was 100 percent effective at preventing severe disease and hospitalization. Vaccine efficacy was consistent across ethnicity and age. In participants 65 years and older, who had not been as well represented in earlier studies, vaccine efficacy was 80 percent. [, accessed 24 Mar]

The interim safety and efficacy analysis was based on 32,449 participants, accruing 141 symptomatic cases of COVID-19. The trial – the largest of its kind for the two-shot regimen – had a 2:1 randomization of the vaccine to placebo.

AZD1222 was well tolerated, and there were no safety concerns related to the vaccine, including the risk of blood clotting. 

For adults of all ages

“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,” said Dr Ann Falsey, Professor of Medicine, University of Rochester School of Medicine in Rochester, New York, US, and co-lead principal investigator of the trial. “This analysis validates AZD1222 as a much-needed additional vaccination option, offering confidence that adults of all ages can benefit from protection against the virus.”

The new data would improve global confidence in the vaccine, which was under scrutiny this month when over a dozen countries, mostly in Europe, temporarily put on hold the use of the vaccine over concerns about possible rare side effects. That was a huge blow to the AZD1222 jab, whose low cost and relatively easy storage and transport requirements are vital to vaccination efforts around the world as new variants spread and the global death toll rises.

No evidence of increased risk

AstraZeneca’s own review of available safety data covering more than 17 million people who have received the jab in the EU and UK found no evidence of increased risk of blood clots in any defined age group, gender, or batch in any country.

The European Medicines Agency (EMA), after a preliminary review, concluded that the vaccine’s benefits in combating the threat of COVID-19 continue to outweigh the risks of possible side effects. The vaccine was not associated with an increase in the overall risk of thromboembolic events in those who receive the jab, the EMA said in a statement. [, accessed 24 Mar]

Following the EMA statement, Germany, France, Italy, Spain, the Netherlands, and at least seven other countries, including Indonesia, cleared the vaccine for use again.

Earlier, the WHO reaffirmed its support for the vaccine which remains a centrepiece of its COVAX vaccine programme.  

An application to the US FDA for emergency use authorization is likely in the coming weeks. The vaccine is already approved in more than 70 countries. Getting the green light from the US regulators would certainly rebuild public confidence and increase vaccine uptake.

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