The impairment of these senses is “just such a weird symptom that doesn’t occur with most other diseases so it’s rarely wrong,” Tim Spector, a professor of genetic epidemiology at King’s College London and a lead author of the study, tells The New York Times.

In the new study, Spector and his colleagues reviewed self-reported data from more than 2.5 million people living in the United States and the United Kingdom. Participants recorded health information on a daily basis, revealing if they were asymptomatic or symptomatic, if they’d been hospitalized, if they had pre-existing medical conditions, and if they had been tested for an active infection of SARS-CoV-2, the virus that causes COVID-19. The users recorded their data on an app, called the Covid Symptom Study, between March 24 and April 21, and in that time, more than 18,000 individuals provided COVID-19 diagnostic test results.

Of the 7,178 who tested positive for coronavirus infection, 4,668—65 percent—reported a loss of smell and taste. About 20 percent of the participants with negative tests reported a loss of smell and taste.

The team analyzed all of the data from UK users to identify independent symptoms that most strongly correlated with COVID-19 and adjusted the results for age, sex, and BMI. An impairment in taste and smell, extreme fatigue, cough, and a loss of appetite were the best indicators of a SARS-CoV-2 infection. Using that information, the team developed a formula to predict if users had the disease or not and applied it to the more than 800,000 app users who reported any symptoms. According to the formula, roughly 140,000 of them probably had COVID-19. The formula was nearly 80 percent accurate at predicting whether or not an app user had COVID-19 based on a positive diagnostic test.

Although it featured prominently in the study, the loss of taste and smell is considered a less common symptom of SARS-CoV-2 infection, according to the World Health Organization. The results of the new study indicate it should be bumped up to one of the most common symptoms, which include fever and cough. In the app, those symptoms ranked fourth and fifth, respectively, as predictors of COVID-19. The study results also imply that a loss of smell and taste could be used to determine who to screen for SARS-CoV-2 infection.

Because of the app’s success in predicting cases, the team suggests it could be a valuable tool for public health officials to use to identify SARS-CoV-2 infections early on and to encourage individuals with those symptoms to self-quarantine, so they don’t spread COVID-19 to others.

“At the moment, we’re mostly gathering data on the tip of the iceberg from those who are really sick and show up at the hospital. But there is a huge iceberg below of people with mild symptoms who we know are major culprits for community spread,” coauthor Andrew Chan, a professor at Harvard Medical School, tells the Times. “We have no ability to track these people at home and that’s a real problem.”

John Brownstein, an epidemiologist and chief innovation officer at Boston Children’s Hospital, agrees. He wasn’t involved with the new app but has used others to track influenza and COVID-19 and says these tools are important for identifying an outbreak before individuals start arriving at hospitals.

“Because we have such a lack of testing, this kind of data is going to give us insights into symptomatology, hot spots and the impact of social distancing,” he tells the Times. “Without this information, how are communities supposed to know we’re on the other side of this pandemic and whether we can reopen?”

Source: The Scientist