Overall, an estimated 1% of people in the San Francisco Bay Area have had Covid-19, while 6.9% of people in New York City have, according to the paper’s authors, who included researchers at the Centers for Disease Control and Prevention and state health departments. In seven of the 10 sites, the estimated number of cases was 10 times the number of reported cases.

The study was based on tests from more than 16,000 people across the 10 sites, but one limitation is that it relies on old data. The San Francisco samples were collected from April 23 to 27, while the New York tests were on blood from March 23 to April 1. The latest tests were conducted in May, and a lot can change during two months in the course of an outbreak. In South Florida, for example, researchers estimated that 1.9% of the population had antibodies to the virus. But that figure is based on samples collected from April 6 to 10, and given the spread of the virus since then in the state, the number now would certainly be some amount higher.

Still, the data reflect what CDC Director Robert Redfield recently said — that true case numbers are 10 times higher than confirmed diagnoses. Confirmed cases in the U.S. stand at more than 3.8 million.