urging governments to wait for international standards on the passports before opening up travel, lest uneven standards lead to unsafe practices or geopolitical gamesmanship.

“A challenge since the beginning has been getting countries to do what’s best for the world instead of what’s best for people inside of their borders,” Dr. Errett said.

Witness the maneuvering within the European Union, whose 27 countries share long borders but have starkly different economic needs and vaccination rates.

Southern European states like Spain and Greece, which rely on tourism, are pushing for the bloc to adopt the documents. German and French officials have expressed reservations, at least for now. Their countries have lower vaccination rates, meaning that travel restrictions would put their residents at a relative disadvantage.

When Britain’s foreign secretary speculated recently that proof of vaccination might be required for pubs and stores, a lawmaker in his own party, Mark Harper, retorted, “I don’t think you want to require people to have to have a particular medical procedure before they can go about their day-to-day life.”

California’s vaccine struggle, over whether to tighten school requirements after measles and whooping cough outbreaks highlighted the state’s low immunization rates, offers a worrying preview.

one-third of Americans, in one poll, predominantly Republicans — are merely hesitant. The push to achieve herd immunity will depend on that third.

One problem: There is no agreement on the primary purpose of a vaccine passport program.

Governments typically talk about them as a way to open up economies. Individuals, as a way to re-enter normal life. Public health experts, as a way to reduce transmissions.

Those goals align, but imperfectly. At some point, the authorities have to prioritize.

Dr. Errett ticked through implementation questions, broadly unknown, that could force an answer. Would you need two doses to get the document or just one? Do Russian- or Chinese-made vaccines qualify? What are the rules for religious or medical opt-outs? Are some activities restricted to card-carriers until herd immunity, just until infections fall below a certain line — or forever?

“We need to be cognizant of the costs and benefits,” she said, and not just to adjust as we go, but for “the precedent we’re setting.”

“We pandemic people,” she said, “have been saying it since the beginning: We don’t expect this to be the last pandemic that we see.”

Matina Stevis-Gridneff contributed reporting from Brussels.

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