Yesterday, the CDC published New COVID-19 Cases and Hospitalizations Among Adults, by Vaccination Status — New York, May 3–July 25, 2021.
The authors of this CDC study did what I suggested the authors of the Mayo-nference pre-print should have done. The CDC authors look at all the data for NY, only excluding people with one dose of the vaccine. As the point is to estimate how well the vaccine works, and you are supposed to get two doses, this is entirely appropriate.
The CDC helpfully breaks down the total data by big demographic groups, and looks for interesting patterns. And there are some. I had guessed that older people might have lower vaccine effectiveness against the Delta variant because of generally weaker immune systems, but at least in New York it is the 18-49 group that has lower effectiveness. This is great news!
As the CDC study carefully notes, what you are seeing here is an un-decomposable blend of virus mutants and behavior changes, but I find it encouraging that the real-world effectiveness scales higher with older people, who have massively increased risk from COVID. I’m glad to be wrong here.
I think this is a way better methodology, and my suggestion to the Mayo-nference pre-print authors is to redo their analysis this way. It is a pre-print, after all.