Explained: Why Apple and Google's virus alert apps had limited success

The virus-tracing effort raises questions about the power of Big Tech to set global standards for public health tools

Sarah Cavey, a real estate agent in Denver, was thrilled last fall when Colorado introduced an app to warn people of possible coronavirus exposures. Based on software from Apple and Google, the state’s smar­tphone app uses Bluetooth signals to detect 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.

Cavey immediately downloaded the app. But after testing positive for the virus in February, she was unable to get the special verification code she needed from the