Millions of people have used this app to log their COVID-19 symptoms — and data suggests the set of symptoms governments initially warned about was too narrow

• Nearly 3.5 million people are using an app daily to report whether they're experiencing symptoms of COVID-19.
• The Covid Symptom Tracker app, built by researchers at King's College London and analytics company Zoe Global, provides real-time data that could help lawmakers decide how to reopen countries in the coming months.
• App data suggests that the symptoms health authorities first described as typical of the virus — fever and cough — were too narrow, leaving out as many as half of COVID-19 patients who never experienced those symptoms.
• Rather, loss of taste and smell is the strongest predictor of whether someone will develop more severe COVID-19 symptoms, according to the data.
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As governments prepare to lift COVID-19 lockdowns across the globe, real-time data from an app with millions of users could provide a critical tool to help policymakers understand the spread of the virus.

Nearly 3.5 million people have been using the Covid Symptom Tracker app to report whether they're experiencing any symptoms associated with COVID-19. The data shines a light on when the number of people experiencing symptoms peaked, the locations of major outbreaks, and which symptoms appear the most common.

The app was created by King's College London and health analytics company Zoe Global, which herald it as the "biggest citizen science project in health that we have ever seen anywhere in the world." It has 3.1 million users in the UK and is now gaining steam in the US, where more than 200,000 people have downloaded it.

Data from the app is already poking holes in some early assumptions about the virus. As recently as a month ago, many health authorities were warning that fever and a cough were the most common symptoms — but app data suggests that roughly half of those infected never experienced those symptoms. The CDC added six more symptoms, including chills, muscle pain, headache, and loss of taste and smell to its COVID-19 page earlier this week.

The loss of taste and smell appears to be the clearest indicator that someone may develop more severe COVID-19 symptoms later on, researchers working on the app told The Wall Street Journal.

In the UK, the number of people reporting COVID-19 symptoms peaked around April 1 and has steadily declined since, according to app data. The country entered lockdown a week prior to the peak.

The data has some shortcomings — including the fact that people over 70, who are particularly vulnerable to COVID-19, are less likely to own a smartphone — but researchers are building algorithms to make data from the app more reliable. The app automatically compares symptoms that users report to baseline data from 30,000 confirmed COVID-19 patients to create a baseline, and researchers told WSJ they're confident at least two-thirds of the data is accurate.

Other tech companies have embarked on similar data-gathering missions. Facebook launched a COVID-19 symptom tracker last week, which will similarly aim to provide health authorities with broad trends on symptoms being reported across the globe.