The Associated Press

OLYMPIA — More than 1 million Washingtonians enabled a coronavirus exposure app in the first four days of the technology going live statewide.

Officials with the state Department of Health said that as of Friday, the number of phones with the WA Notify app is more than 19 percent of the more than 5.8 million adults in the state.

On Monday, the state joined 18 states or U.S. territories that have already enlisted the use of smartphone technology in the ongoing effort to prevent the spread of COVID-19. The technology, created by Apple and Google, is in use in more than 30 countries.

People with iPhones can enable the ‘exposure notifications’ feature that is already in their phone’s settings, and Android devices can download the app, called WA Notify. Use of the app is voluntary and users can opt out at any time. The statewide expansion comes after a monthlong pilot project used by 3,500 participants — including students, faculty and staff — at the University of Washington.

The app uses Bluetooth wireless technology to detect when someone who downloaded the app has spent time near another app user who later tests positive for the virus. Phones using WA Notify exchange random codes, which officials said are completely anonymous, with no location tracking or exchange of personal information.

When someone tests positive for the virus, a health official will ask them if they have WA Notify and will give them a verification code — which is not tied to their identity — to enter into the app. Once entered, the app can determine users who have been near the person who tested positive and notify them of possible exposure. No information about who may have exposed them or where the exposure occurred is included in the notification. The notification will direct people to a website with information on next steps, including how and where to get tested.

After a person tests positive for the virus, a contact tracer gets in touch with the person to determine where they have been and who they have been around in order to ensure that close contacts are notified and told to get tested and self-quarantine.

Officials said that the benefit of the app for is situations where a person who tests positive doesn’t know the people they were around, for example on a bus or at a grocery store. If others nearby also have the app, they will be notified about the potential exposure within 24 hours of the COVID-positive user entering their verification code into their app.

As of this week, more than 172,000 cases have been confirmed statewide and there have been 2,900 deaths.

Nationwide, there have been more than 277,000 virus-related deaths and over 14.2 million confirmed infections.

For most, the coronavirus causes mild or moderate symptoms, such as fever and cough that clear up in two to three weeks, although long-term effects are unknown. But for some, especially older adults and people with existing health problems, it can cause more severe illness, including pneumonia and death.