Does anyone know of a good resource that explains what 'efficacy' means for a vaccine, and what can (and cannot) be extrapolated from it? The reported efficacies are (from what I understand) 1 - (n infections in experimental group / n infections in control group). But then I see people draw wildly varying conclusiona from that, most of which I suspect are wrong (e.g. the relationship between 'efficacy rate' and 'group immunity' after everyone is vaccinated). But despite looking long and hard, I can't anyone explaining in detail what such efficacy rates mean (and don't mean).