People who get the coronavirus vaccine in Los Angeles will be able to keep proof of immunisation in their iPhone's digital wallet.
The record will live in the Apple Wallet, usually used for payment cards, boarding passes, or event tickets.
Officials say it will first be used to remind people to get their second required dose of the vaccine.
But it could also be used as proof of vaccination in the Covid-19 hotspot of Los Angeles.
Bloomberg first reported news of the county's partnership with software firm Healthvana.
The digital card could be used "to prove to airlines, to prove to schools, to prove to whoever needs it," that a recipient has been immunised, the company's chief executive Ramin Bastani told the newspaper.
Los Angeles County, the most populated county in the United States, has a high level of infection of the disease, with nearly 13,000 new cases reported on Tuesday, and 227 deaths - bringing the number of those who have died with the disease to 9,782 to date.
COVID-19 Daily Update:— LA Public Health (@lapublichealth) December 30, 2020
December 29, 2020
New Cases: 12,979 (746,089 to date)
New Deaths: 227 (9,782 to date)
Current Hospitalizations: 7,181 pic.twitter.com/baWng6uejU
The county's vaccine chief Claire Jarashow told Bloomberg that officials were keen to make sure people came back for their second vaccine dose - as the paper document given to recipients is easily lost.
"We just don't have the capacity to be doing hundreds of medical record requests to find people's first doses and when they need to get their second," she said.
Healthvana has a track record of handling sensitive patient data, having delivered millions of HIV test results to US patients in recent years.
But there has been some resistance to the idea of a "vaccine passport", which some fear could be used to deny entry to venues if people are unwilling to share personal medical information.
In the UK, a minister in charge of the vaccine rollout suggested such a system could be used in the near future - before those comments were walked back by a more senior minister, Michael Gove.
Max Van Kleek, Associate Professor of Human-Computer Interaction at the University of Oxford, said the convenience of the system and the fact that it can send a reminder for the second dose "can be very useful to remind people to show up." And the system also makes it far quicker to find patient records.
Another potential advantage is privacy - the relatively simple pass system "can't actively perform surveillance, unlike smartphone apps".
"Passes have the huge advantage of not transmitting data behind-the-scenes... transferring data requires an explicit act of showing the pass. Thus, it's literally in the hands of the end-user," he said.
But there are also some potential problems - such as not having the digital vaccine record on all phones, including Android users.
"If those who have smartphones can more easily prove they've been vaccinated, than this inherently advantages those over others."
And there is no way to prove that the person holding the phone is the same person who received the vaccine, without some other identifier.
Even if those problems are solved, "there is also likely to be a political pushback among those who do not want to be pressured to get vaccinated", Prof Van Kleek warned.
Those people might see this idea as a "hostile approach", designed to withhold access to getting on board a flight, or attending an event.
"This might, in turn, spur conspiracy theories about the use and provision of such technology altogether," he said.