The COVID Symptom Study app that was launched in the UK in March and extended to the USA and Sweden has more than 4 million users, making it the world's largest study of its kind. Users of the app are asked to log their health and any new potential symptoms of COVID-19 everyday. The findings, albeit not peer-reviewed, have greater implications for clinical management of the Coronavirus outbreak and can enable doctors to predict who is most at risk and likely to need hospital care. 

The six distinct types of COVID-19 symptoms are:

1. Flu-like symptoms with no fever
2. Flu-like symptoms with fever 
3. Gastrointestinal
4. Severe level one, fatigue 
5. Severe level two, confusion
6. Severe level three, abdominal and respiratory 

Professor of healthcare engineering at King's College London, Sebastien Ourselin said that the ability to collect big datasets by using the app and apply machine learning to them has profoundly impacted their understanding of the extent and implications of COVID-19 on human health.