Apparently 60% of the population should use these apps in order for them to be effective, which turns into 80% of smartphone users.<p>In Singapore the uptake of the TraceTogether app has only been 12%of the population, and the city ended up in lockdown anyway.<p>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.<p>Source: <a href="https://www.bbc.co.uk/news/technology-52294896" rel="nofollow">https://www.bbc.co.uk/news/technology-52294896</a>