App shows promise in tracking new coronavirus cases, study finds

Zoe Global via The New York Times
The Covid Symptom Study smartphone app.

In the absence of widespread on-demand testing, public health officials across the world have been struggling to track the spread of the coronavirus pandemic in real time. A team of scientists in the United States and the United Kingdom say a crowdsourcing smartphone app may be the answer.

In a study published Monday in the journal Nature Medicine, researchers found that an app that allows people to check off their symptoms was remarkably effective in predicting infections among the 2.5 million people who were using it between March 24 and April 21.

The study, which tracked people in the United States, the United Kingdom, and Sweden, found that the loss of taste and smell, was the No. 1 predictor of whether a person was going to get sick with COVID-19, the illness caused by the coronavirus, followed by extreme fatigue and acute muscle pain.


The researchers were able to predict with nearly 80 percent accuracy if a person was likely to have COVID-19 based on their age, sex, and a combination of four symptoms: loss of taste or smell, persistent cough, fatigue, and loss of appetite.

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Two-thirds of those who later tested positive for the virus — about 15,000 people — had self-reported the loss of taste and smell, the study found. Fever and cough — symptoms that have been considered the most reliable indicators — ranked fourth and fifth on the list.

Because loss of smell and taste is often associated with mild cases of COVID-19, Spector said health officials could use information provided by a surveillance app to encourage participants to quarantine until they were able to get tested.

“The more we collect this stuff and the more we document it properly, the better we can deal with new outbreaks,” said Dr. Tim Spector, a professor of genetic epidemiology at King’s College London and a lead author of the study.

The researchers said they hoped the findings might persuade WHO and other health agencies to modify guidelines that currently rank fever and cough well above loss of taste and smell as symptoms for determining who to screen for COVID-19.


Dr. Andrew T. Chan, a professor at Harvard Medical School and the lead investigator on the study, said a surveillance app could help health authorities identify people who are unknowingly spreading the virus.

“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,” said Chan, who is also chief of clinical and translational epidemiology at Massachusetts General Hospital.

If more widely adopted, the study results could provide public health authorities an inexpensive tool for detecting outbreaks in cities, states, and even individual neighborhoods. Given that the loss of taste and smell appears to be an early indicator of COVID-19, the information, the researchers said, would allow health officials to prepare for a spike of infections and help guide the allocation of scarce resources like ventilators for the most seriously ill, and the personal protective gear needed by medical workers.

As local outbreaks subside, the app can also guide decisions about the easing of lockdowns and social distancing measures.

The researchers said the app did not prompt significant privacy concerns because participants are not required to provide their names and any other personal information, only their ZIP codes.