Apple and Google are just providing APIs.

It up to countries and states to build apps on those APIs.

In the US I think it is by each state not federal/countrywide.

* https://9to5mac.com/2020/05/21/covid-19-exposure-notificatio...

* https://www.wired.com/story/covid-19-contact-tracing-app-fra...

The API is supposed to be available once 13.5 goes live (and it is available in public beta now, I believe). Here is a list of states who are participating with compatible apps: https://9to5mac.com/2020/05/21/covid-19-exposure-notificatio...

TL;DR: Hardly any.

Edit: Whoops, I am out of date, I thought 13.5 was not out yet! Apparently it is, and then some.

From the link above:

> To implement its contact tracing functionality, Immuni leverages the Apple and Google Exposure Notification framework (see Apple’s documentation and Google’s documentation). This allows Immuni to overcome certain technical limitations, thus being more reliable than otherwise would be possible.

13.5 went live on May 20. They just released 13.5.1 yesterday to patch the unc0ver jailbreak.

https://support.apple.com/en-us/HT211214

Ah my bad, thanks for the correction. I'm still at 13.4 and I haven't been overtly refusing upgrade notifications, so I thought it hadn't been released yet.