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 US 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 US 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 US,” 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.” These were people who showed up at a doctor’s office or clinic with symptoms. Most people with Covid-19 likely never sought treatment of testing for it. “The US-wide ILI surge appeared to peak during the week starting on March 15 and subsequently decreased in numerous states the following week; notable exceptions are New York and New Jersey, two of the states that were the hardest hit by the epidemic, which had not started a decline by the week ending March 28,” the team wrote. The researchers could not count every single case, so they ran a series of calculations to make sure their data fit in with what’s known about state populations and about the annual flu epidemic, as well as with the hard data that was collected from actual testing of coronavirus patients. They also took into account growing evidence that people started avoiding hospitals, clinics and doctor’s offices once it was clear there was a pandemic, and after pandemic lockdowns started. “If 1/3 of patients infected with SARS-CoV-2 in the US sought care, this ILI surge would have corresponded to more than 8.7 million new SARS-CoV-2 infections across the US during the three-week period from March 8 to March 28, 2020,” the researchers wrote. Cases fell after that. “We saw this huge peak that ended on March 22 in most places,” Silverman told CNN. Cases have been on the decline since then, he said – but the data the team is collecting does not include the past two weeks. The team is now working to try to get closer to real-time surveillance of the pandemic. The data from the CDC comes in about two weeks after people make their doctor visits. They hope their approach – called syndromic surveillance – could complement data collected from actual testing. “In a dream world, everyone who comes in would have a test. We would be able to get a full scope of the pandemic,” Washburne told CNN.