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.

The two rivals are collaborating to develop a contact-tracing system for release as soon as next month.

However, their scheme relies on specific wireless chips and software that are missing from hundreds of millions of smartphones that are still in active use, particularly those that were released more than five years ago.

“The underlying technology limitation is around the fact that there are still some phones in use that won’t have the necessary Bluetooth or latest operating system,” said Ben Wood, analyst at CCS Insight. “If you are in a disadvantaged group and have an old device or a [basic] feature phone, you will miss out on the benefits that this app could potentially offer.”

Public health agencies are being asked to incorporate the Silicon Valley companies’ technology into their own apps, which will notify people if they have come into contact with an infected individual and urge them to self-isolate. The more people who opt in to using the app, the more successful the system will be.

The particular kind of Bluetooth “low energy” chips that are used to detect proximity between devices without running down the phone’s battery are absent from a quarter of smartphones in active use globally today, according to analysts at Counterpoint Research. A further 1.5 billion people still use basic or “feature” phones that do not run iOS or Android at all.

“In all, close to 2 billion [mobile users] will not be benefiting from this initiative globally,” said Neil Shah, analyst at Counterpoint. “And most of these users with the incompatible devices hail from the lower-income segment or from the senior segment which actually are more vulnerable to the virus.”

Smartphone penetration and uptake of particular devices varies widely around the world. In the UK, media regulator Ofcom said last year that about 80 percent of adults own a smartphone. However, Mr. Wood estimates that only around two-thirds of adults would have a compatible phone.

“And that’s the UK, which is an extremely advanced smartphone market,” he said. “In India, you could have 60-70 percent of the population that is ruled out immediately.”

Counterpoint Research is more optimistic, estimating that 88 percent compatibility in developed markets such as the US, UK, and Japan, while about half of people in India would own the necessary handset.

“Apple and Google working together to enable the use of Bluetooth technology to help government agencies identify and arrest the spread of the virus is a step in the right direction,” Shah said.

© 2020 The Financial Times Ltd. All rights reserved. Not to be redistributed, copied, or modified in any way.