In a 2009 paper that was published in Nature, researchers famously showed that Google searches related to the flu had been closely tracking weekly data on influenza rates from the Centers for Disease Control and Prevention. Researchers used these search terms to build a model to try to help detect epidemics before the official data was collated.

Although the model did work initially, it struggled during the 2009 H1N1 flu pandemic. The problem was that flu was in the news so often, many people were searching for flu not because they were feeling symptoms but because they were feeling curiosity or fear. Concern about flu and Google searches about flu were more in the air than actual flu.

Recently, scholars have produced new methods to improve Google-based disease prevalence modeling and helped revive the influenza-tracking project. They have found that it is crucial to key in on the types of searches that are most likely to be reports of symptoms rather than searches related to news.

These tools are being used right now by researchers studying how searches might track Covid-19. Searches like “I can’t smell” are particularly useful because the form of the query suggests that someone may have the disease, whereas other queries related to loss of smell may instead suggest curiosity in the topic.

There is another way we can use search data during this pandemic: to better understand symptom patterns of the disease. Our understanding of the progression of symptoms of the disease is still developing. It took until March 20 for widespread reports of the relationship between Covid-19 and loss of smell to surface, even though it now appears to be among the most common symptoms.

There is already some evidence that clues to this symptom were evident earlier in search data. Joshua Gans, a professor at the Rotman School of Management at the University of Toronto, found that searches for “non sento odori” (“I can’t smell”) were elevated in Italy days before the symptom was reported in the news. Iran also saw an enormous rise in searches related to loss of smell weeks before media reports of the symptom became common.