A few percent of Americans have been infected with SARS-CoV-2. In a study of 30,000 people, enough people will get infected within a few months to give good sta

A few percent of Americans have been infected with SARS-CoV-2. In a study of 30,000 people, enough people will get infected within a few months to give good statistics. At the current rate of confirmed cases in the US, you'd expect about 10 people in the study to get infected each day (fewer if half of them are vaccinated and the vaccine works). Without a placebo group, how will you measure effectiveness and safety? You need to monitor a statistically equivalent group of people to determine the difference in rates of infection and disease, and to look for side-effects.