An illustration of a hand holding a smartphone with a coronavirus cell on it.

Apple and Google announced Friday that they are working together on tools that would allow smartphone users to see whether they have come in close contact with someone with COVID-19. Early versions will be ready next month, according to a news release.

The system would allow for widespread contact tracing, a disease-monitoring measure that health officials say is critical for slowing the spread of the pandemic. If people know they were recently exposed to an infected person, they can self-isolate in order to avoid giving the disease to someone else.

On Friday, CDC director Robert Redfield told NPR that “very aggressive” contact tracing will be required before many businesses can bring back their workforces, and that location data collected by cellphones could be a key to putting a program in place. Several widely distributed academic proposals also call for unprecedented surveillance measures to accompany the loosening of social distancing protocols.

But existing proposals to track peoples’ locations through their smartphones have raised big privacy concerns, especially if they involve sharing that information with the government. And experts say it's not clear how effective those tools would be in practice.

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The new system from the two tech giants addresses some of these worries. It would rely on Bluetooth, a technology that allows a device to communicate with other devices nearby. If people choose to participate, their phones will keep a 14-day log of other devices they come into proximity with, whether that's in a grocery store, on a bus, or along a city street.

Then, if someone tests positive for COVID-19 and chooses to report the result to a contact-tracing app, other users who crossed paths with that person in the past two weeks will be notified so that they can get tested or self-isolate.

Most of the data processing that will determine whether you've been near an infected person will take place on the phone, and the two tech companies say they will not collect user locations or other information that can identify an individual. This is meant to ensure that the information can not be repurposed, say, for advertising or law enforcement.

That’s in stark contrast to contact-tracing systems that have been set up in China, Taiwan, and South Korea, where governments integrate location data with detailed personal information from official databases to keep tabs on residents.

“Privacy, transparency, and consent are of utmost importance in this effort,” Apple and Google said in the joint statement. To that end, they have made some documentation explaining how the system will work publicly available.

The first contact-tracing apps based on this new infrastructure could appear in May, built by public health organizations. Later, Apple and Google say, some of the functionality will be incorporated directly into the software of their phones.

“This is a huge step forward to assist the public health community with contact tracing while preserving user privacy,” says Albert Gidari, director of privacy at the Stanford Center for Internet and Society. “It has the advantage of scale and interoperability while giving users the choice to participate.”

But both technology and health experts say questions remain about the effectiveness of any app-based contact-tracing system.

To start, Bluetooth can be imprecise: It can be hard to know how close two Bluetooth-enabled devices were to each other, and variations in phone hardware can affect the range and reliability of a signal. Apple tells CR that it's studying ways to determine how far away one device is from another based on a Bluetooth signal.

And though experts say contact tracing is a vital part of managing the coronavirus pandemic, it’s only as useful as the extent of actual COVID-19 testing, an area where the U.S. is lagging. If people can't get tested for the disease, they won’t know to submit their diagnosis to a tracing system.

“If this prompts you to get tested and we haven’t solved the testing issue, you may be in a cul-de-sac,” says Andrew Schroeder, director of analysis at Direct Relief, a research organization that is tracking the spread of COVID-19. Plus, not everyone who tests positive will decide to share that information with an app.

Health officials say that the lack of testing, plus the fact that some people have COVID-19 without developing symptoms, means there are probably droves of unreported cases out there. That means you could opt in to the program and never get an alert about close contact with a carrier, even if you really have been exposed to the virus.

“I worry that could lead to a false sense of security and make people overly confident about going out and interacting with others,” says Justin Brookman, CR’s director for consumer privacy and technology policy.

Another potential problem with contact-tracing technology is being addressed simply by the fact that this is an Apple-Google collaboration. If only a few people participate in any particular program, the results will be woefully incomplete and potentially misleading.

But Apple and Google make the operating systems for almost every smartphone on the market. That means that the 81 percent of Americans who have a smartphone should have easy access to the program.

However, the 19 percent of Americans who don't have smartphones are concentrated in some low-income and rural areas. That means there would be glaring holes in the data this system gathers, Schroeder says, even if everyone who can opt in does so.

The only way to include everyone in a contact-tracing system is to assign teams of investigators to track a person’s recent movements and interactions after they’ve tested positive for coronavirus, experts say. 

That’s how Singapore, for example, has attacked the issue. But scaling up personalized tracing from a city-state to a country of almost 330 million would require enormous resources—even if Americans were willing to embrace such aggressive government surveillance.