France was one of the first Western countries to build an app to track exposure to the coronavirus, but sluggish takeup rates could limit its usefulness in preventing a second wave.

Source: French Contact-Tracing App Struggles with Slow Adoption. It Isn’t Alone – WSJ

What many do not understand is that it requires a lot of people to have a smart phone and to install and use the app for a phone to detect potential coronavirus exposures.

To illustrate – in most modern countries about 80% of adults have a smart phone (plus or minus). When we adjust the figure for young kids who do not have a smart phone, about 2 in 3 people have a smart phone or 63%.

If half of all smart phone users install a contact tracing app, then we have about 32% have the app. In order to detect a contact, you need to be close to someone else with the same app. The probability of that occurring is .32 x .32 or 10%. Thus, what seems like wide spread adoption yields a potential of detecting just 10% of possible Covid-19 contacts.

And this probability does not take into account the problems with contact tracing apps:

  1. Incapable of detecting contacts across time (someone sits on a bus seat, coughs, gets up, leaves, and then you sit there)
  2. Difficulties with multi-path radio signals that result in false signal strength readings (this is common)
  3. Incapable of detecting when there is a barrier between contacts. For example, you are on a bus stopped at a bus stop while the driver takes a break. Someone with Covid-19 standing outside the bus results in alerting your phone that you just had a contact. Except you really did not due to the physical barrier.
Coldstreams Skeptic