Poland has now joined Bulgaria, the Czech Republic, Croatia, Denmark, Germany, and Greece in backing the program, which will allow travelers to demonstrate their vaccination and infection status.
The seven states have started issuing the certificates, which use QR codes to allow people to verify their details with the relevant border authorities.
No personal data is exchanged or retained during this process, while paper versions are also available for those without smart phones, the Commission said.
It added that more member states will join in the coming weeks before the scheme launches fully on July 1.
Brussels has called on all EU states to participate and to prepare their own national systems to issue, store, and verify the certificates.
The certificates will allow travelers within the bloc to avoid quarantine if they are fully vaccinated, have recovered from the virus, or have tested negative.
The Commission has advised member states to adopt a 72-hour validity period before travel for PCR tests and 48 hours for rapid antigen tests.
Meanwhile, the EU is reportedly negotiating with the US about the entry of American travelers into the bloc under the scheme.