A new study suggests that 8.7 million Americans contracted COVID-19 in March but over 80% went undiagnosed.

Researchers studied data from the Centers for Disease Control and Prevention (CDC) and found that the number of people who visited doctors or clinics with flu-like symptoms peaked in March.

According to CNN, the researchers said that only 100,000 cases were officially reported during that time period because testing was sparse. The U.S. still reported only 2,3 million cases as of Monday but the actual number, according to the calculations of the researchers, would be closer to 8.7 million cases if all the infected individuals who sought medical care during the March peak period had been tested and identified.

“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.,” Justin Silverman of Penn State University, Alex Washburne of Montana State University and their colleagues from Cornell University, wrote.

They published their findings in Science Translational Medicine in which they explained that the number of patients with ILI reported by medical personnel during the month of March was much higher than the normal seasonal average of flu-like cases. The surge coincided with soaring COVID-19 counts across the country. The CDC asks doctors to report all cases of patients who come in with fever, cough and other influenza symptoms so that the agency can track seasonal flu epidemics.

“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,” Silverman and his colleagues wrote. The team hopes to continue their surveillance of the pandemic as more data from the CDC comes out, according to CNN.