Competing interests: In March and early April, the authors developed a prototype of a geolocation-based contact tracing app. They do not report having, and have never had, any commercial interest in developing the app (and are no longer working on the project). The authors will freely share their work with any public health agency. Colin Merkel reports that he is a software engineer who worked at Google until June 2019. Google’s parent company, Alphabet, makes the Android phone operating system, and Google produces mapping services. Colin Merkel now works for an unrelated education start-up company. He did not receive any compensation for this work. No other competing interests were declared.