Comment by nradov

CAIR2 is specific to California. Every state registry has some technical differences in HL7 V2 Messaging support but most are reasonably close to the CDC/AIRA spec.

https://repository.immregistries.org/resource/hl7-version-2-...

The CDC IZ Gateway can make it somewhat easier to work with multiple state registries.

https://www.cdc.gov/vaccines/programs/iis/iz-gateway/overvie...

In practice the C-CDAs obtained from providers sometimes include immunization section entries so you can pick up some records that way as well