Why are Covid-19 contact-tracing apps producing so many false positive results? Technical issues may be to blame
Countries around the world have been setting up Covid-19 contact tracing apps for Android and iOS devices, with Northern Ireland rolling out the StopCOVID NI app several months ago and the UK's NHS app being delivered more recently. The apps will alert users if they have been in close proximity with other users who have tested positive for Covid-19 and will tell them to get tested.
All of the apps function roughly the same way, by using your phone's bluetooth connection to detect nearby devices from other people using the app. The system tries to judge whether you've been within 2m of someone who tested positive for more than 15 minutes, catching potential spreading on public transport or in shops and restaurants.
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Technical problems plagued the UK's initial effort to develop a contact-tracing app on its own, as many mobile phones weren't able to reliably detect nearby devices. The UK scrapped its initial app and built a new one that uses a special contact tracing API developed by Google and Apple, the same API that's used in almost all contact tracing apps in use around the world right now.
It turns out that bluetooth isn't a reliable way to measure the real distance between two devices, as signal strength and quality varies heavily between different devices. Anyone who has used bluetooth speakers in their home will know that the signal can also be blocked by walls, scattered by other people getting in the way, and disrupted by certain electronic devices, and can be reflected by metal inside a vehicle.
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As the apps convert signal strength into distance, they can misjudge the distance and miss contacts. On the other end of the scale, they can miscalculate that a phone is within 2m and generate a false positive result.
Wired UK has written an interesting piece breaking down some of the problems with bluetooth and the solutions that are being used to combat them. This includes a massive study that calibrated bluetooth strength by measuring it on over 12,000 handsets, which can be used in apps to get a more accurate distance calculation.
Source: Wired UK
