> The number of smartphone users in India is around 500 million out of a nearly 1.4 billion population. Most of the smartphones are also cheap low end models th

> The number of smartphone users in India is around 500 million out of a nearly 1.4 billion population. Most of the smartphones are also cheap low end models that likely won’t support this app You don't need 100% of anything to control an outbreak. If the app allows you to catch transmission events from the 22.3% or whatever of smartphone users who have compatible devices, and that pushes R0 down below 1.0, then you win. Every little bit helps. Nothing in isolation is going to do it. I'd be curious about that Ars article you mention. I can't believe it would make the statement in that way, because that's not really a correct understanding of the problem.