How Covid-19 Contact Tracing Works on Your Phone

Our smartphones are set to play a significant role in helping navigate our way out of the coronavirus pandemic, with countries and companies around the world preparing their own apps as part of a track-and-trace system to keep infection levels low.

Google and Apple don't work together on much, but they're working together on this: a set of underlying protocols inside Android and iOS that are able to speak to each other, even while your phone is in your pocket.

The first fruits of these efforts are now live on Android phones and iPhones—here's how to find these settings on your phone, and what they actually do.

The Track-and-Trace Technology

Photograph: Apple
What Apple and Google have developed isn't an app in itself—rather it's an application programming interface (an API), plus some other fundamental technologies, that other apps can plug into. When you load up a website with a Google Maps widget on it, that is using a Google Maps API, and the Covid-19 tracking tools work in the same way.In other words, Apple and Google have done the groundwork, making sure that health apps can talk to each other across Android and iOS and get access to the features they need. It's now up to countries (and states ) to develop the apps that plug into these foundations and provide the actual front-end interface for users. (If indeed they decide to—some agencies are working on completely bespoke systems of their own.)
A crucial part of this underlying framework is access to Bluetooth signals. Bluetooth is perfect for low-energy wireless transmission that can run in the background of your phone, without draining the battery excessively. (It's used for wireless headphones, car stereos, and the like.)

In this case, your phone will be logging other phones it comes into contact with, assuming both your device and the others are running a Covid-19 tracking app that's been fully enabled (which is why public support is going to be so important). These logs don't include any identifying information about you; they use random numerical ID codes that change frequently and get trashed completely once they're older than 14 days (the incubation period for Covid-19).

Based on what we know so far, the apps will be able to log the length of time you've been in contact with each person (or rather each individual phone), and how far away you were, judging from the strength of the Bluetooth signals. Any contact that's less risky (such as briefly passing someone on the street) will be ignored.

Finding the Settings

Very few Covid-19 tracking apps are out in the wild yet, but the features that Apple and Google have worked on are now live. Besides the settings that you'll find in future tracking apps, you can enable or disable "exposure notification" logging at the operating system level as well—it's a completely opt-in system.