[nradov]

Right, the data quality is usually crap. Beyond the issues you mentioned, patients often switch providers or health plans and their data doesn't get migrated. In the USA at least there is no centralized national repository for that data so the further back you try to go the more likely the data will just be missing (or incorrectly coded). In theory there are interoperability APIs and national networks to solve this problem but in practice a lot of systems still aren't properly connected.

For vaccinations specifically the CDC Immunization Gateway can be a good place to start. Most states also maintain their own immunization registries that can be queried through standard HL7 V2 Messaging and/or FHIR APIs if you have the appropriate permissions.

https://www.cdc.gov/iis/iz-gateway/index.html