The CDC's vaccine data is all wrong

Or look at Maryland, where they say 99.9% of seniors have gotten at least one shot. About 15% of Maryland lives in Prince George’s County, where 98.6% of seniors are vaccinated. In Baltimore County, home to about 9% of the state, it’s 96.1% of seniors. About 585,000 of Maryland’s six million residents live in Baltimore City (an independent city-county, not part of Baltimore County), and the CDC says that 85.5% of the city’s seniors are vaccinated.

In other words, the states seem to aggregate to a lower number than the national figure. And the county numbers seem to aggregate to a lower number than the state figure.

The inconsistency strikes me as odd, but I won’t dwell on it too much because the answer to the riddle “Why is the CDC’s vaccination count so bad?” turns out to be that actually nobody is counting. In the footnotes, they explain that they receive data that has been de-identified for privacy purposes, and consequently, you may be reported to them as two separate individuals if you get different doses from different providers. They also explain that not everyone has county of residence information, which is why the county-level counts aggregate up to a lower number than the state-level count. Then, in a methodological choice that I think was a mistake, they top code everything at 99.9% on the theory that “this cap helps address potential overestimates of vaccination coverage due to first, second, and booster doses that were not linked.”