Europe seems poised to set the global standard for vaccine passports, now that European Commission President Ursula von der Leyen has signaled that vaccinated Americans will be allowed to travel to the continent this summer.

Why it matters: Opening up travel to vaccinated Americans will bring new urgency to creating some kind of trusted means for people to prove they've been vaccinated.

The big picture: There will probably never be a single credential that most people use to prove they've been vaccinated, for every purpose.

• But the EU's system will help set a standard for a proof of vaccination that's both easily accessible and difficult to forge.
• The U.S. is being closely consulted on the European passport, so any future American system will likely use similar protocols.

Details: Informal mechanisms like simply asking someone whether they're had a shot can suffice in many situations. A system for international travel will likely be far more stringent. And there's a wide middle, too.

• Other activities that don't need the same rigorous standards as international travel could rely on the CDC's vaccination cards; options like a printed QR code, similar to what's been proposed by PathCheck; or a digital QR code, like the ones created by CommonPass or the Vaccine Credential Initiative.
• There may be some state-issued credentials, like the Excelsior Pass in New York.
• A national credential is theoretically possible, and could be linked to the biometric information that already exists on many chipped passports — the World Health Organization is working with Estonia to develop something along those lines — but that would meet steep political resistance in the U.S.

The bottom line: The world of vaccine passports is almost certainly going to end up as a mishmash of different credentials for different activities, rather than a single credential used by everybody for everything.

Go deeper: Americans will likely have to navigate a maze of vaccine "passports"