There were ~120,000 new cases of Delta on December 3rd. 0% were Omicron. There were ~140,000 new cases of COVID (Delta + Omicron) December 20th. Some unknown percentage were Omicron. If NowCast is right, and 73% of the new cases ending the week of December 18th were Omicron, that means Delta went from infecting 120,000 people per day to something like 38,000 (27% of 140,000) per day. That makes no sense. By contrast, if total case numbers were 440,000 it would make sense, because it would imply that Delta was still chugging alongside Omicron.
I’ve been so puzzled by the CDC announcement from the moment I saw it — the idea that Omicron was suddenly 73% of new cases, but with only a small increase in total cases just makes no sense. I felt like I was taking crazy pills since pretty much all the high profile folks I follow were not discussing this odd fact. It’ll be interesting to see what actually happened.