Updated: 10:43 AM EDT Apr 5, 2021 Starting Monday, residents of two Massachusetts cities can download an app that is supposed to supplement COVID-19 contact tracing efforts. Somerville and Methuen are participating in the pilot testing of the MassNotify app, which is designed to notify users who were in close contact with another user who later tests positive for the virus. "If you choose to enable MassNotify, your phone will share anonymous Bluetooth codes, which do not contain personal information or location data, with other phones nearby," Somerville officials wrote in a description of the app. "If you test positive, you can choose to anonymously alert other MassNotify users so they know about the possible exposure. If you have recently been near someone who tests positive for COVID-19, and they share their result using MassNotify, you may get a private exposure alert on your phone. This alert will tell you that you may have been exposed to COVID-19 and will send you information to help you quarantine and get tested."It is built on technology Apple and Google developed for using a cellphone's Bluetooth system to log potential exposures. It's a tool that's already been included in operating system updates for iOS and Android devices.State health officials have been working on the technology for months. In November, the Department of Public Health began seeking bids for projects to test and evaluate the technology. Several countries and other states have already adopted the technology or something similar. New York, for example, has the COVID Alert NY app.