Apple and Google’s joint effort to help public health authorities track the spread of COVID-19 will begin testing today, the companies said. The companies will deliver an initial version of their exposure notification API to a small number of developers working on apps for public health organizations and test it over the next several weeks. The API is expected to be released more broadly in mid-May.

The companies announced their rare collaboration earlier this month. The project, which users will be asked to opt into, will use Bluetooth to track potential exposures to confirmed cases of COVID-19. Smartphones will record instances in which they have been close to other phones for an extended period of time without recording location data. When a person is diagnosed with COVID-19, the API will allow public health agencies to quickly inform other people who may have been in contact with the patient, based on the data stored on their phone. Information about patients’ identities or locations is not shared with Apple or Google.

Apple is releasing a beta version of Xcode 11.5 and the third beta of iOS 13.5 today. The former includes developer tools for building iOS apps that incorporate the API, and the latter includes the code needed to run the future apps from public health authorities. Google has released similar updates through Google Play Services and Android Developer Studio.

Public health authorities will have control over what they consider a likely exposure, using the API to account for the approximate distance the phones were from one another and how long they spent in proximity. Both are calculated on users’ phones and not shared with the companies.

Apple and Google will release sample code on Friday that’s intended to show developers how to build apps using the system, they said. In the second phase of the project, expected within the coming months, exposure notification will be built directly into iOS and Android.