Limitations of Contact Tracing with Smartphones

In my last post, I wrote about a new COVID-19 contact tracing app for residents of Los Angeles. In this post, I’d like to discuss two technological limitations of contact tracing smartphone apps.

The first technological limitation is that of false positives. As discussed in an article by The Brookings Institution, “False positives (reports of exposure when none existed) can arise easily. Individuals may be flagged as having contacted one another despite very low possibility of transmission—such as when the individuals are separated by walls porous enough for a Bluetooth signal to penetrate.”

I believe that this is exactly what happened to me immediately after I installed SafePass. Although I did not receive an exposure notification, SafePass did inform me that I had a Bluetooth contact.

This happened about an hour after I had installed SafePass. I was the only person in our household who installed SafePass, and I hadn’t left home during that time. Therefore, I conclude that SafePass must have detected a Bluetooth signal from one of my 2 neighbors that share (thin) walls with me in our 10-unit condo complex. Had my neighbor contracted COVID-19 and documented it in SafePass, I would have received a false positive exposure notification.

The second technological limitation is a foreground/background issue. That is, contact tracing apps may not work properly unless they are running in the foreground (i.e., the active/visible app). As discussed in this article, iOS apps have limitations when running in the background. While protecting privacy and battery life, iOS also limits the utility of contact tracing apps unless users remember to bring them back to the foreground. On the other hand, Android apps are allowed to operate while they’re not in the foreground; read the article for more details about the rationale for the different behavior on iOS and Android devices. In any case, on iOS devices, apps in the background are put into a suspended state and might look like this in the app switcher:

Apps in a suspended state may still be able to use Bluetooth services is they declare to iOS that they want to do certain things while in the background. However, if another app or iOS itself needs RAM, iOS can terminate suspended apps so that they no longer run at all.

There are lots of variables at play here. Compounding the issue is that I have an old iPhone 6 Plus that has only 1 GB of memory, so suspended apps on my phone are frequently terminated upon launching other apps. Remembering to regularly re-launch SafePass is impractical and unlikely to happen on a consistent basis for most users, even if they understand the foreground/background issues described above.

A potential solution to the iOS foreground/background issue is Exposure Notifications Express which Apple introduced with iOS 13.7. Exposure Notifications Express can provide contact tracing without the need to install a dedicated contact tracing app. However, it must be supported by a public health authority, and users must enable COVID-19 Exposure Notifications in their settings. According to this article, users who opt in will be notified when their local public health authority adopts Exposure Notifications Express.

In conclusion, smartphone-based contact tracing is not a panacea, nor will it replace the labor-intensive work of manual contact tracing. Rather, to put things into perspective, the use of contact tracing apps will be a supplement to a myriad of approaches that we need to implement simultaneously. Those include but are not limited to social distancing, mask wearing, handwashing, sufficient testing, contact tracing (manual and via smartphones), developing effective treatments, and ultimately the administration of safe and effective vaccines.