Factbox: Key insights from U.S. trial data on AstraZeneca's COVID-19 vaccine

FRANKFURT (Reuters) - AstraZeneca and Oxford University’s coronavirus vaccine received a major boost on Monday as data from a large trial showed it was safe and effective, potentially paving the way for its emergency authorisation in the United States.

The trial results, primarily generated in the United States but also in Chile and Peru, included a 79% efficacy in preventing symptomatic COVID-19, and showed no increased risk of a rare form of blood clots that has caused concern in parts of Europe.

The following lists key insights from the trial as it contrasts with previously known data from a trials run in Britain, Brazil and South Africa.


The U.S. trials, designed and run by AstraZeneca, were simpler, with two standard doses administered at a four-week interval, while the data presented late last year from trials organised by Astra’s development partner Oxford University featured a range of four to 12 weeks between shots.

While the inconsistent intervals in the Oxford trials were due to vaccine production delays, it allowed for an analysis of how a delayed booster shot might influence efficacy.

Britain’s Medicines and Healthcare products Regulatory Agency concluded at the time that a 12-week gap between doses was the key to achieving the highest efficacy, of around 80%.

FILE PHOTO: A vial of the AstraZeneca coronavirus disease (COVID-19) vaccine is seen, as vaccinations resume after a brief pause in their use over concern for possible connection to blood clots, in Turin, Italy, March 19, 2021. REUTERS/Massimo Pinca

Oxford’s trial also included a group that received an initial half dose followed by a full standard dose as a booster shot, which was the result of a measurement error.

Though AstraZeneca initially said the non-standard dosing pattern held early promise of being 90% effective, the approach was disregarded by regulators and no longer pursued by Astra.


Oxford’s trials in Britain, Brazil and South Africa did not have enough cases of COVID-19 in elderly participants to show the shot also works in that age group. In the U.S. trials, 20% of participants were aged 65 years or over.

While Britain’s regulator found it safe to assume it worked in the elderly based on blood analysis, some European countries including Germany initially only cleared the product for those younger than 65.

The U.S. trial data presented by Astra on Monday showed for the first time in a controlled trial that the shot also had a strong effect in the elderly. “In participants aged 65 years and over, vaccine efficacy was 80%,” it said.


While AstraZeneca did not break down the data into specific variants, the company’s statement has reassured some experts on the product’s usefulness against mutated, potentially more infectious forms of the coronavirus.

The efficacy number falls short of figures of well over 90% reported by the Pfizer-BioNTech alliance and by Moderna for their respective vaccines in December, but comes after more infectious variants of the virus had spread around the globe.

Analysts at Jefferies said the trials were likely conducted under a higher prevalence of such variants than prior studies by rivals, one of the reasons for describing it as better than expected, it added.

Reporting by Ludwig Burger; Editing by Jan Harvey