An app by the Virginia Department of Health enables COVID-19 Exposure Notifications for your phone. Should you install it if you’re not in Virginia?

Virginia’s COVIDWISE app is now available for iOS and Android. It’s the first app in the US to use the Exposure Notifications System — a set of APIs use your phone to detect folks nearby and notify you anyone you’ve been near tests positive for COVID-19.

Alabama, North Dakota, Wyoming and several other states are working on apps that also use ENS, but until your state comes out with its own, the Virginia Department of Health’s app is a worthwhile download even if you’re outside of Virginia.

To understand why, you need a little bit of background about how exposure notifications work.

Apple and Google designed the Exposure Notification System to work on any phone with recent software.

The system has two parts. First is exposure — proximity over time. If you choose to install and enable an ENS app, your phone pings any nearby phones that also have ENS enabled. All of this happens phone-to-phone via Bluetooth. ENS-enabled phones send random identifiers that change several times an hour, so it’s anonymous. Your phone remembers any identifiers it received in the past two weeks. All of this happens directly on the phone, no location data is involved, and no data leaves the phone.

But none of this happens until you install an app like COVIDWISE that uses ENS. The phone-to-phone pinging works the same with any ENS app. As long as you install one and grant it permissions, your phone will start pinging others. Thus the more folks that install apps like this, the better the coverage will be.

Where do notifications come from?

From there, it’s up to state health departments to handle the notifications part of ENS. Anyone who gets a positive COVID-19 test result in Virginia gets a secret number to key into the app. That makes their app send the list of pings it’s received in the past two weeks to a notifications server. Every COVIDWISE app downloads that list every day or so, and the app notifies you if it sees its identifier on that list.

With the the COVIDWISE app, notifications are closely tied to the Virginia, the FAQ explains:

The app will continue to exchange anonymous keys with other COVIDWISE users that have Exposure Notifications enabled wherever you are, even outside of Virginia. However, any user that chooses to submit a positive diagnosis must be verified with the Virginia Department of Health to receive a PIN to submit in the app. This makes the app much less effective outside of Virginia.

COVIDWISE App FAQ

There has been some movement toward a national system for ENS notifications by the Association of Public Health Laboratories. And some of the newer ENS apps may already take advantage of this. But for now, notifications are limited to folks who use the COVIDWISE app.

Even so, it’s a worthwhile download. The Washington Posts’s Geoffrey A. Fowler agrees: I downloaded Covidwise, America’s first Bluetooth exposure-notification app. You should, too.

The app requires a phone running iOS 13.5 or Android 6 or later.