An EU-wide Covid certificate for easier travel came into force on Thursday, just in time for Europe’s busy summer vacation period — but the highly infectious Delta variant is already threatening to curtail its use. The EU document — essentially a QR code available on smartphones or on paper — shows whether the bearer is vaccinated with one of the European Union’s approved jabs (from BioNTech/Pfizer, AstraZeneca, Moderna or Johnson & Johnson), or whether they have recovered from an infection or recently tested negative. Under EU law, the certificate is meant to do away with the need for quarantines or further testing when travelling between the EU’s 27 countries or four associated European nations (Iceland, Norway, Switzerland and Liechtenstein). All EU member states were connected to the digital certificate system on Thursday except Ireland, which was hit by a cyber-attack targeting its health service in May and plans to have it operational on July 19. But a surge in the Delta variant, first detected in India and now quickly gaining ground elsewhere, could trigger an “emergency brake” provision suspending the certificate’s acceptance. Germany already has a ban on incoming travellers from Portugal, where the Delta variant has become dominant. Only its own citizens or residents are allowed in from Portugal, and they must quarantine for two weeks upon arrival. Berlin’s decision has raised Brussels’ ire, with EU Justice Commissioner Didier Reynders saying on Wednesday that “we should avoid travel bans” within the EU and stressing that Germany should have consulted with its partners first.