The half dose was an 'experimental endpoint'. Always planned but not seen as critical. FDA rules now stipulate you can only register on primary endpoints declared before the trial starts. I suspect AZ have argued that the accelerated nature of the process should take this into account.

I don't think that gives an accurate picture of what happened. Even AstraZeneca themselves are calling the half-dosage a mistake. Their US trial of that vaccine does not currently contain a half-dose arm [1], though it likely will be amended to do so. As best I can read it, the dosage amount was planned but it was intended to represent the "full" dosage and not to be a half-dosage, but was updated when they noticed unusually mild physical symptoms in people receiving the vaccine. [2] I haven't seen anything to support the idea that a half-dosage was an intended endpoint, whether secondary or otherwise. Do you have a source for that?

[1] https://clinicaltrials.gov/ct2/show/NCT04516746 [2] https://www.reuters.com/article/uk-health-coronavirus-astraz...

"Controlled" doesn't mean what you think.