As many as a billion mobile phone owners around the world will be unable to use the smartphone-based system proposed by Apple and Google to track whether they have come into contact with people infected with the coronavirus, industry researchers estimate.

The figure includes many poorer and older people—who are also among the most vulnerable to COVID-19—demonstrating a “digital divide” within a system that the two tech firms have designed to reach the largest possible number of people while also protecting individuals’ privacy.

Apple’s iPhones and devices running on Google’s Android operating system now account for the vast majority of the 3.5 billion smartphones estimated to be in active use globally today. That provides a huge potential network to track infection, with surveys suggesting widespread public support for the idea.