No, 50 % means the infection rate (or whatever you're actually measuring) in the vaccine group is half that of the control group. In your example the infection

No, 50 % means the infection rate (or whatever you're actually measuring) in the vaccine group is half that of the control group. In your example the infection rate are exactly the same, giving an efficacy of 0 % (± whatever confidence interval the size of your trial and placebo groups gives you). See e.g. https://blogs.sciencemag.org/pipeline/archives/2020/09/21/the-vaccine-protocols https://blogs.sciencemag.org/pipeline/archives/2020/09/21/th...