http://oivwjeplttrmbwfeysvsxt3e4shatvjj3led3bmnwfwa7kz62qvy4dyd.onion
When the certificate is checked, the QR code is scanned and the signature verified. Each issuing body (e.g. a hospital, a test centre, a health authority) has its own digital signature key. All of these are stored in a secure database in each country.