Virus alert apps powered by Apple and Google have had limited success.

When Apple and Google collaborated last year on a smartphone-based system to track the spread of the coronavirus, the news was seen as a game changer. The software uses Bluetooth signals to detect app users who come into close contact. If a user later tests positive, the person can anonymously notify other app users whom the person may have crossed paths with in restaurants, on trains or elsewhere.

Soon countries around the world and some two dozen American states introduced virus apps based on the Apple-Google software. To date, the apps have been downloaded more than 90 million times, according to an analysis by Sensor Tower, an app research firm. Public health officials say the apps have provided modest but important benefits.

But Natasha Singer of The New York Times reports that some researchers say the two companies’ product and policy choices have limited the system’s usefulness, raising questions about the power of Big Tech to set global standards for public health tools.

Computer scientists have reported accuracy problems with the Bluetooth technology. Some of the app users have complained of failed notifications, and there has been little rigorous research on whether the apps’ potential to accurately alert people of virus exposures outweighs potential drawbacks — like falsely warning unexposed people or failing to detect users exposed to the virus.