County-level CDC and DHS vaccination statistics for Wisconsin vary in important ways

The Federal Centers for Disease Control (CDC) and the Wisconsin Department of Health Services (DHS) both independently collect COVID-19 vaccination statistics. Their statistics for the state of Wisconsin vary. Statewide, the CDC counts 3,469,836 fully vaccinated individuals (as of 2021-12-01), while the DHS counts 3,268,304 (as of 2021-12-02). This gap of 201,532 is too large to be explained by small differences in the reporting period.

Proportionally, the differences are even larger at the county level. For instance, according to the CDC, 76% of Bayfield County residents are vaccinated, but the DHS places this figure at 66.4%. The opposite pattern is shown in neighboring Douglas County, where the CDC vaccination rate is 39.9%, and the DHS rate is 58.9%.

The DHS acknowledges differences between their data and the CDC, saying “Data on this page may differ from data reported on the CDC COVID Data Tracker due to the fact that data may be updated on different schedules and reflect data ‘as of’ different dates or times of day. There may also be a delay between the time a vaccination record appears in the the state system and when it is received by CDC.”1 Likewise, the CDC website states, “Data presented here use standard metrics across all United States counties. However, data on this page may differ from data on state and local websites. This can be due to differences in how data were reported or how the metrics are calculated. For the most accurate and up-to-date data for a specific county or state, visit the relevant state or lcal health department website.2 However, simply deferring to the DHS isn’t the best option in Wisconsin, because the CDC receives reports from some agencies which don’t share records with the state government.

There isn’t consensus about which set of numbers is best to use. The Milwaukee Journal Sentinel uses the DHS statistics in their vaccine tracker, while the New York Times uses the CDC numbers. As I dug into the details of each dataset is tabulated, I discovered that each has strengths and limitations. The two datasets need to be combined to create the most accurate picture of vaccinations at the county level in Wisconsin.

County level discrepancies

The map and table below show the differences between county-level CDC and DHS estimates downloaded on 2021-12-02.

In most counties, the CDC vaccination count is slightly higher than the DHS count. In a few counties, the CDC count is significantly higher. And in a handful of counties, the DHS count actually exceeds the CDC. All of these latter counties have something specific in common. They all border Minnesota.

I believe there are at least two reasons for this set of discrepancies. The CDC counts more vaccinations overall because they receive vaccination reports from more agencies than the DHS. However, the DHS does a better job of matching vaccine recipients to their home county. Each of these sources of error affect particular counties differently.