Whether they realize it or not, about 200 million people in the United States now likely have access to a Covid-19 digital vaccine card.

The digital pass known as the SMART Health Card is voluntary and minimal by design to protect personal information. It has a person's name, date of birth and the dates and brands of vaccination doses, all contained within a type of scannable bar code known as a QR code.

And after a relatively quiet start, it has built momentum in recent months as more states and companies have signed on, making it something of a de facto national digital vaccine card.