Google has updated its Passes API to enable a simple and secure way to store and access COVID vaccination and test cards on Android devices.
Starting today, developers from healthcare organizations, government agencies and organizations authorized by public health authorities to distribute COVID vaccines and/or tests will have access to these APIs to create a digital version of COVID vaccination or test information. This will roll out initially in the United States followed by other countries.

Once a user stores the digital version of the COVID Card to their device, they will be able to access it via a shortcut on their device home screen, even when they are offline or in areas that have weak internet service.
To use this feature, the device needs to run Android 5 or later and be Play Protect certified. Installing the Google Pay app is not a requirement to access COVID Cards.

More info here

Example COVID Cards from Healthvana, a company serving Los Angeles County

Example COVID Cards from Healthvana, a company serving Los Angeles County