Anon pondering here…there've been near constant blood drives in the US coinciding with pandemic start.

1. All blood is tested (HIV, Hep-B, Rh factor, etc.), results aggregated at federal level.

2. Chance increased blood drives coincides with population surveillance to monitor for vax uptake?

3. Could aggregate or de-identified data be FOIA'ed?

Could be broken down by demographics, zip code, etc. Check for specific tests run or results of specific tests, getting data before 2018 as control group data.