Stopping COVID-19 before there's a vaccine means preventing sick people from spreading it.
While total lockdowns are one way to do that, South Korea and China have dramatically reduced infections in part by contact tracing, the practice of tracking those with whom infected people have come into contact.
Contact tracing is an old practice, and a well-known part of the anti-virus public health cocktail.
According to the World Health Organization (WHO) and CDC, it's generally been done by hand.
Interviewers talk to people who get sick, tracking their movements and interactions, and then call around to the people they've interacted with, creating a map of possible infections.
The MIT Technology Review estimates that if the US does traditional contact tracing by hand, we'll need 100,000 workers to do it.
Digital contact tracing replaces at least some of those interviews with technology.
Especially in South Korea and China, it's been an effective way to keep infections down.
(South Korea is finally down to zero local infections.) But the successful approaches used elsewhere rely on a level of trust in authority and giving up privacy, which may not be acceptable in the individualistic United States.
Apple and Google just released APIs for contact-tracing apps, which now need to be developed by third parties and opted into by users.
There's no word on who will be developing these apps in the US and whether there will be any coordination between the app developers to share data.
Several different approaches are being used.
Your mobile phone carrier can track your location at all times, by analyzing cell tower connections.
In South Korea, when someone is diagnosed with COVID-19, those tower hits are being shared with local governments, which combine them with CCTV footage, credit card receipts, and interviews, and broadcast the results on the web and through text messaging.
In China, businesses and public transit systems have QR code scanners outside, and everyone entering must scan a QR code from an app on their phone.
All of these motions go into a central government database, which figures out who's been near people who were sick.
In Taiwan, anyone who is already supposed to be under home quarantine has their phone checked by authorities, remotely, and the police get an alert if the phone is turned off for more than 15 minutes.
The HaMagen app in Israel checks your phone's GPS history against a central government database of anonymized GPS coordinates, which the health ministry built using more traditional contact-tracing methods.
If you've crossed paths with any infected people, it alerts you, but does not directly alert the authorities.
Bluetooth-based contact-tracing apps such as Singapore's TraceTogether rely on phones running the app in the background, searching for nearby Bluetooth devices also running the app—that's how Apple's AirDrop works.
Phones can roughly determine the distance between each other based on Bluetooth signal strength; recent iPhones can also use their U1 ultra-wideband chips to figure out their proximity to each other.
Unlike the network and QR-code-based solutions, Bluetooth-based apps drain your phone's battery.
What a Bluetooth app does with its data is up to the individual app developer.
TraceTogether stores a data log which gets extracted and used by the government if a user is diagnosed with coronavirus, to find who they may have been around.
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Contact tracing requires knowing who's sick.
If it's not mandatory to use an app, people need to opt in.
In Singapore only 12 percent of people opted in to the territory's contact-tracing app.
If it relies on an app, people need devices that run the app.
Nineteen percent of Americans don't have smartphones, according to Pew, which means they may not be able to run an app.
If Apple and Google can't share data, we have another problem: the US is almost evenly split between Android and iOS.
South Korea gets around these problems by using text messaging and carrier-based location tools that work with all phones.
In China, everyone uses WeChat and Alipay, two "super apps" that run on almost every phone and can be ordered to be compatible by the government.
Effective contact tracing can be far more invasive than most Americans are willing to accept.
This Seoul metro government web page lists the movements of every person diagnosed with COVID-19 in the city.
While it's anonymized, it's probably possible for people to figure out these people's identities.
There are no contact-tracing apps in the US right now, so there's no way to tell whether they're working or not.
We expect the first ones to start appearing this month.