I think it’s entirely from different vaccination rates. Check the stats for 2020 only ie before vaccines were widely available. Because there were indeed no significant red-blue state differences then. The paper I linked above includes all US states in its analysis as well as 43 other countries.
Edited to add: try going through the US states data in this report, written March 2021 ie before the Delta wave with differential vaccine rates.
I think it’s entirely from different vaccination rates. Check the stats for 2020 only ie before vaccines were widely available. Because there were indeed no significant red-blue state differences then. The paper I linked above includes all US states in its analysis as well as 43 other countries.
Edited to add: try going through the US states data in this report, written March 2021 ie before the Delta wave with differential vaccine rates.