It's called based on null hypothesis. Well, accurately say majority of business as usual, more accurately say at least 80% business should remain as usual. The data from Diamond Princess already told us 80% of her 3,500 people are immune to covid-19, 20% got infected (but 80% of the infected are either have mild symptom or no mild symptom). The rough picture is clear to me, detailed and accurate picture is still not clear though.

BTW, Drug testing is a clinical trial thing (e.g. double blind), it is not a modeling thing