Anosmia, or losing your sense of smell, affects more than your nose. Because taste is linked to smell, you lose both senses. It's a relatively common side effect of the common cold and seasonal influenza. (Up to 40% of people who get those viruses experience anosmia, according to one research project.)
With thousands of anecdotal reports of loss of smell linked to the coronavirus pandemic, some people think it's an early sign of the virus. But it might not be, say some doctors.
"Why is smell getting so much attention?" Steven Munger, director of the University of Florida Center for Smell and Taste in Gainesville, asked National Geographic. "People are scared, and we're trying to understand this disease. We're trying to reach for things to help us recognize COVID-19 as early as possible."
Because it's so common, this sign may not be all that useful as a diagnostic tool for coronavirus. Plus, not everyone who experiences anosmia has the virus. Symptom data collected through an app created by researchers at King's College London showed that some people who tested negative for coronavirus still reported a loss of smell.
However, the U.S. Centers for Disease Control and Prevention (CDC) recently added recent loss of taste or smell to its official symptoms list, which now also includes chills (and chills with repeated shaking), muscle pain, headache and sore throat. So far, the World Health Organization (WHO) has not added "inability to smell" to its list of symptoms.
Even if coronavirus does turn out to be a cause of anosmia, it can't be used to diagnose whether you have it or not. Just like a fever or a sore throat, loss of smell can be caused by a number of viruses. The only way to know if you have the virus for sure is to get tested.
Using search data to understand virus spread
But that doesn't mean anosmia and other symptoms aren't useful data points or that symptoms should be disregarded.
As economist and data scientist Seth Stephens-Davidowitz wrote in his recent New York Times opinion piece, data is worth tracking and might indeed give useful information during a fast-moving health crisis. As he points out, there seems to be a strong link between loss of smell and the spread of coronavirus to new locales — and a relationship between the number of Google searches for the term and areas where it has hit more severely, like New York City and New Orleans.
Importantly, there are ways to look at search data and differentiate between those who are sick and those who are just curious about a particular symptom: They would likely type in something like "loss of smell coronavirus" whereas someone who is actively experiencing the symptom would type in "I can't smell." Searches for the latter phrase were higher in Italy and Iran in the days before the virus took hold in those hard-hit countries.
While not necessarily a way to diagnose the illness, you can also still "use these searches to try to find places where many positive cases are likely to have been missed," writes Stephens-Davidowitz. As an example, he points to Ecuador, where there have been low numbers of cases in official reports, but overall data points to higher possible rates of the virus. Tracking data may also be a good way to understand symptom patterns of coronavirus.
In fact, in the video above, Dr. Alfred-Marc Iloreta from Mount Sinai Health System discusses the symptom and suggests patients takes the conservative approach if they suddenly lose their sense of smell.
Eye pain: A new symptom?
And there may be some symptoms still to be connected to to coronavirus — or ruled out. As Stephens-Davidowitz wrote: "There is also some evidence for eye pain as a symptom of COVID-19 from searches in other parts of the world. Notably, searches for eye pain rose above fourfold in Spain between the middle of February and the middle of March and rose about 50 percent in Iran in March. In Italy, searches for "bruciore occhi" ("burning eyes") were five times their usual levels in March.
Does that mean there's a new COVID-19 symptom? Stephens-Davidowitz readily admits "Not necessarily," but the point of data-crunching isn't real-time disease tracking, but learning more about how we might be able to track diseases in the future.
Using search data might not help us immediately, but it could expand what we know about the virus' spread and the speed it travels. And it serves as sample data for the future. As we learn more, we can better track and understand the next inevitable virus.
Editor's note: This story has been updated since it was first published in April 2020.