Already posted. Go back and read. The state databases that exist are incomplete and almost all of them have opt out provisions. A few states are opt in.
There is no national database, despite the fact that you might wish there was one.
If that’s the case, then travelers coming from States that don’t mandate IIS participation are going to have a tough time proving vaccination status. The various COVID-19 passport apps pretty much all depend on the QR code which is part of the digital record.