CDC has tainted epidemiological data by issuing guidance to exempt vaccinated people from testing unless symptomatic. Vaccinated asymptomatic cases were simply not counted for a period of time ending in late June 2021. I can't find the date when they issued the faulty guidance for a precise time interval, but any study comparing vaccinated / unvaccinated cases relying on US epidemiological data including May/June 2021 must sadly be considered tainted.

https://www.nytimes.com/live/2021/07/28/world/covid-delta-va...