Another one: don’t program your own AB testing framework! Every time I’ve seen engineers try to build this on their own, it fails an AA test (where both versions are the same so there should be no difference). Common reasons are overly complicated randomization schemes (keep it simple!) and differences in load times between test and control.
This is a rewriting of history. Before we ever had results from the COVID vaccine trials, the FDA declared that the primary goal of the trials was to prevent any symptomatic infection in the first place. From June 2020: https://www.fda.gov/media/139638/download.