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.

SafePass: A Contact Tracing App for Los Angeles

I previously wrote about the Apple-Google partnership on a contact tracing infrastructure using bluetooth connections on smartphones, and I provided more detail about how exposure notification is designed to work.

Since then, many regions of the U.S. and around the world have developed their own contact tracing applications, some with the Apple-Google infrastructure, and some with other methods. There are debates about privacy, and some people prefer not to use contact tracing apps for fear that their data will be used for nefarious purposes. While I think there are legitimate privacy concerns, in my opinion the potential benefits of widespread adoption of smartphone-based contact tracing outweighs the risks. Furthermore, we as a nation have not been adapting well to the pandemic, whereas other countries have gotten the virus under control, using contact tracing as part of a multifaceted approach to mitigating both the health, social, and economic effects of the pandemic.

The City of Los Angeles recently announced a partnership with Citizen, the developer of a mobile app called SafePass that provides contact tracing capabilities for smartphone users throughout Los Angeles County. SafePass will allow users to self-report their COVID-19 symptoms and receive exposure notifications and alerts from the Los Angeles County Department of Public Health. This enables people to get tested following exposure notification and is an important tool that can be used to prevent further spread of COVID-19. Citizen asserts that contact tracing data collected by the app is anonymous, private, encrypted, and deleted after 30 days. If you receive an exposure notification, you will not know who exposed you—you will only know that you were in proximity to someone who later tested positive. Here’s a brief overview of its features:

According to this article, “An Apple rule concerning contact tracing apps forced the company to spin SafePass off into a standalone app. Android users need to download the Citizen app to access the features for now, but the company says a standalone SafePass app will be available in the Play store in the next month.”

I encourage everyone in Los Angeles (and other areas where it has been implemented) to download SafePass and report symptoms. It should only take a minute of your time each day, and it could save lives.

Apple-Google Contact Tracing - Exposure Notification

I previously wrote about the partnership between Apple and Google to develop a common infrastructure to support contact tracing. In short, this is how it is envisioned to work:

According to this article, the contact tracing feature will be called Exposure Notification, and here are some interesting details:

• It will only be available to be used by apps from public health authorities
• Apple and Google can turn off Exposure Notification on a regional basis
• User consent will be required for them to share their anonymous data
• Public health authorities will be able to see anonymous Bluetooth beacon data for those who have tested positive for COVID-19 and those that receive exposure notifications
• Apple and Goole will not monetize contact tracing data

I look forward to seeing public health authorities develop contact tracing apps. Given that it sounds like multiple public health authorities will be developing apps, I infer that the apps will not be at a federal level (e.g., CDC) and that they will be provided by state or local health departments. However, that is unconfirmed. In any case, the network effect will apply here. Similar to how everyone has a social and moral obligation to adhere to social distancing measures, I assert that we will all have a shared obligation to enable Exposure Notification on our mobile devices.

A Smartphone-based Approach to Contact Tracing

Contact tracing is a laborious process that is used to identify and reach out to people how have had contact with someone with a confirmed case of an infectious disease. As part of an effort to contain an outbreak, those contacts are isolated or quarantined. If the contacts develop into confirmed cases, then contact tracing is performed on their contacts too.

I’d like to clarify some terms to put things into perspective. An outbreak is when an infectious disease occurs in unexpectedly high numbers but is limited to one geographic area. When the infection spreads to multiple areas, it is called an epidemic. When the infection spans multiple countries or continents, it is called a pandemic.

Contact tracing is a containment strategy for a disease outbreak. When an infectious disease becomes an epidemic or pandemic, contact tracing is not feasible, and we switch to mitigation strategies such as school closures, limiting restaurants to take-out or delivery, and other social distancing measures. The United States is predominantly addressing COVID-19 through mitigation strategies. However, if we get the disease sufficiently under control and can begin to relax social distancing measures, we will need to ramp up contact tracing to focus once again on containment.

The Margolis Center for Health Policy at Duke University released guidance on a COVID-19 containment strategy that addresses testing, surveillance, contact tracing, isolation, and quarantine. As described in this NPR article, contact tracing is a resource-heavy and time-consuming process.

It was announced today that Apple and Google are working together on a contact tracing infrastructure using smartphones. Here are identical press releases from Apple and Google. More information is available via both technical documentation and a layperson illustration.

The first phase of the collaboration will include APIs that will enable data sharing across Apple’s iOS and Google’s Android devices. This would allow app developers to implement the APIs and share data between the apps and health authorities. The second phase will include a Bluetooth-based contact tracing platform that, according to The Verge, would detect signals from nearby phones at 5-minute intervals and store those connections but not their physical locations. If one person develops COVID-19, it would be possible to determine, based on those connections, who else might have been recently exposed.

An important aspect of this approach that I alluded to is that unlike GPS that tracks actual locations, this Bluetooth-based approach only tracks other smartphone users in their proximity but does not know the physical location of the phone. This might be a critically important aspect of the design of the solution that would allay the fears of people who feel strongly about protecting their privacy. I look forward to seeing the implementation of both phases of this collaboration, and the timing of this collaboration is good because I am optimistic that in the coming weeks/months we can shift from mitigation back to containment.