Wanger said it took a long time to build up all these core enabling technologies: the API for Apple and Google devices to talk to each other, the database of encrypted codes that all states share, and the open-source software states can copy to make their own apps. But now that they’re all in place, it typically only takes four to six weeks for a state to get an app off the ground.

The key question now is how many people will download the apps. Researchers found that if just 15% of the population uses an exposure notification app, it can cut Covid-19 infections by 8% and deaths by 6%.

Correction: Not all US state apps use the Association of Public Health Laboratories database—the apps from Alabama, Virginia, Nevada, and Guam do not. Additionally, phones send a list of all the codes they’ve generated to a central database, not all the codes they’ve received.

This story has been updated with estimated downloads of the New York and New Jersey exposure notification apps.

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