A new study suggests that as many as 8.7 million Americans came down with coronavirus in March, but more than 80% of them were never diagnosed.

A team of researchers looked at the number of people who went to doctors or clinics with influenza-like illnesses that were never diagnosed as coronavirus, influenza or any of the other viruses that usually circulate in winter.

There was a giant spike in these cases in March, the researchers reported in the journal Science Translational Medicine.

“The findings support a scenario where more than 8.7 million new SARS-CoV-2 infections appeared in the U.S. during March and estimate that more than 80% of these cases remained unidentified as the outbreak rapidly spread,” Justin Silverman of Penn State University, Alex Washburne of Montana State University and colleagues at Cornell University and elsewhere, wrote.

Only 100,000 cases were officially reported during that time period, and the U.S. still reports only 2.3 million cases as of Monday. But there was a shortage of coronavirus testing kits at the time.

The team used data collected from each state by the Centers for Disease Control and Prevention for influenza-like illness. The CDC uses this data to track the annual seasonal flu epidemic. It asks doctors to report all cases of people coming in for treatment for fever, cough and other symptoms caused by influenza.

“We found a clear, anomalous surge in influenza-like illness (ILI) outpatients during the COVID-19 epidemic that correlated with the progression of the epidemic in multiple states across the U.S.,” Silverman and colleagues wrote.

“The surge of non-influenza ILI outpatients was much larger than the number of confirmed cases in each state, providing evidence of large numbers of probable symptomatic COVID-19 cases that remained undetected.”