Apple and Google are to work together to create contact tracing technology that aims to slow the spread of the coronavirus by allowing users to opt into a system that catalogs other phones they have been near.

The rare collaboration between the two Silicon Valley companies, whose operating systems power 99% of the world's smartphones, could accelerate usage of apps that aim to get potentially infected individuals into testing or quarantine more quickly and reliably than existing systems in much of the world.

Such tracing will play a vital role in managing the virus once lockdown orders end, health experts say.

The companies said they started developing the technology two weeks ago to streamline technical differences between Apple's iPhones and Google's Android that had stymied the smooth operation of some existing contact tracing apps.

Under the plan, users' phones will emit unique Bluetooth signals if they have tracing technology turned on. Phones within about six feet can record anonymous information about encounters.

If a person tests positive, the system can send an encrypted list of phones they came near to Apple and Google, which will trigger alerts to potentially exposed users to seek more information.

Public health authorities would need to sign off that an individual has actually tested positive before they can send on the data.

The logs will be scrambled to keep infected individuals' data anonymous, even to Apple and Google, the companies said.

Their technology will not track the GPS location of users, they added.

The set-up aims to address privacy concerns that have forestalled aggressive mobile location tracking and other surveillance measures aimed at preventing new outbreaks.

"To their credit, Apple and Google have announced an approach that appears to mitigate the worst privacy and centralization risks," said Jennifer Granick, surveillance and cybersecurity counsel for the American Civil Liberties Union.

Apple and Google plan to release software tools in mid-May to contact tracing apps endorsed by public health authorities, with the two companies approving use of the tools.

However, Apple and Google also plan to release software updates in the coming months so that users do not have to download any apps to begin logging nearby phones.

Dana Lewis, a lead developer of contact tracing app CoEpi, said it and other apps have been collaborating on open source Bluetooth tools for several weeks but would swap in the new official tools next month.

"This is a fundamental building block for apps like CoEpi," Lewis said.

Scott McNabb, an Emory University professor who was a CDC epidemiologist for 20 years, said widespread access to tests as well as training on the new apps for healthcare workers would be required before the phone tools can be effective.

It is unclear how many public health authorities are prepared to embrace digital contact tracing.

"Contact tracing is something that hasn't been able to get done in the U.S. because of the lack of testing, but it can be an effective approach once we get this (virus) under control more," he said.

Governments worldwide have been scrambling to develop or evaluate software meant to improve the normally labor-intensive process of contact tracing, in which health officials go to recent contacts of an infected person and ask them to self-quarantine or get tested. 

While health experts have credited extensive testing and contact tracing with slowing the spread of the virus in nations such as South Korea, privacy experts have questioned whether users in the United States and Europe would adopt digital contact tracing.

Several health technology experts have said the involvement of Apple and Google would be a massive boost to their efforts, as contact tracing apps from the Massachusetts Institute of Technology and others struggled to work across competing operating systems.

The companies' system may use data on the strength of Bluetooth signals to estimate how closely users came into contact with infected people and for how long, which could help estimate the risk they caught the virus.

It was unclear whether the move by Google and Apple would generate global coverage with the technology. Google said the technology will be distributed as part of its Google Play services, which are not available in China or on Android devices that do not use Google's official version of the operating system.

Apple will distribute the technology as an update to its operating system, which many users enable automatically but not all.

Al Gidari, a Stanford University law school lecturer and previously long-time external counsel to Google, described the plans "as exactly the kind of scale we need, coupled with the privacy sensitivity we want."

"This isn't a substitute for testing - you need to know who has it - but it produces actionable results so people can act responsibly, self-isolate and reduce anxiety in the community as a whole," Gidari said.