How does rate of community spread impact efficacy calculations? The placebo and treatment group should be operating in similar areas w/ similar case rates. If t

How does rate of community spread impact efficacy calculations? The placebo and treatment group should be operating in similar areas w/ similar case rates. If there were 10,000 cases in placebo and 1,000 in vaccine, how is that different from 1,000 in placebo vs 100 in vaccine? Not read out to the same efficacy.