The Zoe Health Study, formerly the COVID Symptom Study, is a health research project of British company Zoe Limited (formerly Zoe Global limited) which uses a mobile app that runs on Android and iOS. The app was created in 2020 in response to the COVID-19 pandemic, in a collaboration between Zoe, King's College London, Guy's and St Thomas' Hospitals with funding granted by the UK government until April 2022. The purpose of the app was to track COVID-19 symptoms and other salient data in a large number of people, to enable epidemiological results to be calculated. Timeline.
https://en.wikipedia.org/wiki/Zoe_Health_Study