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Can Your Phone Tell You if You've Been Exposed to COVID-19?

Find out which states are using the exposure notification app

woman with Covid-19 app looking at her phone

kzenon/Getty Images

En español | As the number of new COVID-19 cases in the United States spikes to more than 200,000 daily, the need to rapidly discover who has had certain interactions with whom has again raised the question: Could contact tracing using smartphones make you safer?

Back in May, Apple and Google started working on the underlying technology to create apps that would alert people whenever they were exposed to someone infected with COVID-19. The alerts wouldn't identify the person, preserving privacy. But they would be able to warn you about exposure to strangers even in places such as supermarkets and gas stations — something no human contact tracers can do.

Now close to half of the states have launched Apple iOS and Google Android apps based on this technology. If you live in a state where health officials are working with software developers to provide information about those who have tested positive, you’ll probably see a featured app that mentions or refers to COVID exposure notifications on your iPhone or Android phone. (If you don’t see a featured app, search for “Covid exposure” and your state.) Joining is voluntary and anonymous but open only to state residents age 18 and older.


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Available on iPhones and Android phones, the software technology is officially known as the Exposure Notifications System. It does not track your location using GPS but rather acts as a proximity alert using the low-power wireless Bluetooth feature on most phones. The New York-based nonprofit Brennan Center for Justice considers apps built on this platform to have among the best privacy protections for users’ data.

Whenever you come near someone else using the same app, the phones trade secret codes. Later, if it turns out that you spent more than 10 minutes within 6 feet of anyone who ended up testing positive for COVID-19, you'll be sent an alert and be informed about what to do next. For example, you might be advised to self-isolate or contact your physician.

Is it trustworthy?

Because the software relies solely on information from government authorities, people can’t prank the system by, say, falsely claiming they tested positive and then triggering alerts to scare others. Only people who receive a special code from their local health department can enter a positive result into the app.

Since no national plan has been created for contact tracing, each state is deciding whether to create its own app. That means you’ll see different messages and differently named apps for exposure notifications depending on where you live.

What is close contact?

Remember 6-15-24. If you've been within 6 feet of an infected person for 15 minutes or more in total over 24 hours, the federal Centers for Disease Control and Prevention considers that close contact. Someone who has been exposed won't necessarily come down with COVID-19, but the virus is contagious for 48 hours before a patient has any symptoms or tests positive.

In Alabama, the app is called GuideSafe. In New York, it’s the COVID Alert NY. In Wyoming, it’s called the Care19 Alert.

The kind of information that the apps offer also varies, but the basic principle is the same. For example, if you get a notice in New York — usually a phone call — that you tested positive, you'll also get a code from the health department that you then can enter into the app to send the information anonymously to anyone you might have been near.

While the app works best when you leave it running in the background on your phone, you don't have to leave the app on all the time. You can turn it off and on whenever you want.

As of Dec. 16, at least 22 states, the District of Columbia and two U.S. territories were using contact tracing apps, mostly based on the Apple-Google technology: Alabama, Arizona (phased rollout across the state), California, Colorado, Connecticut, Delaware, the District of Columbia, Hawaii, Maryland, Michigan, Minnesota, Nevada, New Jersey, New York, North Carolina, North Dakota, Oregon (phased rollout with statewide availability expected in January), Pennsylvania, Rhode Island, South Dakota, Virginia, Washington state and Wyoming, plus Guam and Puerto Rico. (On iPhones, Colorado, Connecticut, the District of Columbia, Maryland and Washington state have enabled an opt-in to Exposure Notifications in the phones’ device settings but have no separate app.) And in the absence of statewide apps, Georgia Tech, the University of Illinois Urbana-Champaign and the City of Santa Fe in New Mexico are using apps in their communities.

A Massachusetts app is on track to be developed. Oklahoma expected its app to be rolled out in November, but it has not been released yet. Wisconsin’s app is expected to come out later in December.

South Carolina’s health department initially committed to creating an app for its state — SC Safer Together available in both the Apple App Store and on Google Play — but state legislators blocked the state health department and other state agencies from using cellphone apps for contact tracing because of lawmakers’ security and privacy concerns. Utah debuted its own app in April but turned off its Bluetooth and GPS location tracking by July.

Will it work everywhere?

If you travel outside of your state using your local COVID tracing app, it may work in coordination with some other nearby state apps. For example, apps from New Jersey, New York and Pennsylvania are compatible with each other.

So New Yorkers will get alerts if they travel to one of those states or if someone from one of those states travels to New York and exposes them. However, different companies have created many of the apps scheduled to debut and won't necessarily trade information with other state-supported apps.

Some critics worry that these apps may create a false sense of security and emphasized that a lot of people have to opt in for them to be effective. In California, 4 million users downloaded and signed up for CA Notify on Dec. 10– 11, its first two days available, but officials need a lot more of the state’s 39.5 million residents to say yes for it to work well, according to the California Department of Public Health.

And epidemiologists, such as researchers at Oxford University, say that while having 60 percent of people using an app would be ideal, only a small percentage of smartphone owners would have to join to give health departments information about new hot spots. More important: The researchers point out that at least one infection — and, therefore, one possible death — would be averted for every one to two people who opt to use the software. So a few thousand users could save thousands of lives.

Is this a flawless system?

The underlying Apple/Google technology has had hiccups in some countries, such as England and Wales, where apparent alerts that seemed to indicate a person was exposed to someone with COVID-19 and should self-isolate were sent out. But the messages had been intended only to indicate the app was working properly.

Furthermore, some concerns over privacy led to recommendations from police departments in the United Kingdom that officers do not download the COVID app to their work phones. Since then, police there are being told to use the app on their personal smartphones instead.

Some copycat apps also have popped up online. The best practice is to stick only with smartphone apps that use the Exposure Notification System and that your local health department supports. If you see conflicting information online — don't trust user reviews — contact your local health department for confirmation.

As with any technology, contact tracing using a smartphone app could help make you safer. But you'll still have to be vigilant by wearing a face mask, staying at least 6 feet from others and routinely washing your hands.

Editor's Note: This article was originally published on November 9, 2020 and is now updated with new information on coronavirus exposure apps.

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