It's an intuitive idea: an app that provides proof that a person has received a coronavirus vaccine.

Plenty of technologists are working to make it a reality. Companies of all sizes have been pouring in resources: Microsoft, major airlines, Ticketmaster, prominent nonprofits, security companies, tech startups and blockchain companies are all taking hacks at what some call vaccine passports. Apple and Google have participated in discussions about how to create digital Covid-19 vaccine certificates, experts said, but they haven't announced plans.

But behind the scenes, the realities of medical records, privacy concerns and the virus itself mean such products are unlikely to be widely available in the coming months, experts said.

"This is something that almost no one can focus on right now," said Rebecca Coyle, executive director of the American Immunization Registry Association, a membership organization for state and local vaccine registries. She said digital Covid-19 certificates may seem like "a nice shiny object" but might not be a reality for many months.

The challenges underscore how the pandemic has laid bare the gap between what technologists hope to accomplish and the stark realities of responding to a nationwide crisis. While all states and some cities keep vaccine databases, few of them so far have been willing or able to embrace vaccine apps.

And with some Americans set to start receiving Covid-19 vaccines as early as this month, the proof that they got their shots will come on an older technology: paper.