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.