Apparently 60% of the population should use these apps in order for them to be effective, which turns into 80% of smartphone users.

In Singapore the uptake of the TraceTogether app has only been 12%of the population, and the city ended up in lockdown anyway.

Making those apps opt-in, i.e. optional, is taking a big risk. At the same time deploying a system that works at such scale in such short time isn't easy.

Source: https://www.bbc.co.uk/news/technology-52294896