They never did a proper phase 3 trial. Instead they did two half-trials, with different doses. Initially they pretended this was done on purpose, and claimed

They never did a proper phase 3 trial. Instead they did two half-trials, with different doses. Initially they pretended this was done on purpose, and claimed 90% efficacy: https://www.bbc.com/news/health-55040635 https://www.bbc.com/news/health-55040635 Later it came out this was an accident and cover-up, spun as a lucky discovery: https://www.wired.com/story/the-astrazeneca-covid-vaccine-data-isnt-up-to-snuff/ https://www.wired.com/story/the-astrazeneca-covid-vaccine-da... https://www.reuters.com/article/us-health-coronavirus-astrazeneca-dosing/dosing-error-turns-into-lucky-punch-for-astrazeneca-and-oxford-idUSKBN28327Q https://www.reuters.com/article/us-health-coronavirus-astraz... Even now, the lower bound of 95% C.I. around efficacy with two doses is 63% which is quite low, relative to the headlines: https://www.ox.ac.uk/news/2021-02-02-oxford-coronavirus-vaccine-shows-sustained-protection-76-during-3-month-interval https://www.ox.ac.uk/news/2021-02-02-oxford-coronavirus-vacc... I'm not a troll, just a critic of a lab and a U.K. government that fudged some data because they needed a PR win. https://www.bloomberg.com/news/features/2020-07-15/oxford-s-covid-19-vaccine-is-the-coronavirus-front-runner https://www.bloomberg.com/news/features/2020-07-15/oxford-s-... "I hope we can improve on those timelines and come to your rescue.” - Sarah Gilbert That kind of hubris made it hard to admit process errors and low efficacy rates.