[A] study, published [July 21] in JAMA Internal Medicine [determined that the true number of U.S. coronavirus cases could be anywhere from six to 24 times higher than what’s reported. It] relied on serological tests — blood screens that search for antibodies to the virus and that determine whether someone was previously infected. They are different from diagnostic tests, which only detect people who currently have the virus, called SARS-CoV-2.
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.
[T]he 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.
The data underscore two other points: that testing in the U.S. is not capturing the full scope of the outbreak, and that even hard-hit communities are not close to reaching a herd immunity threshold — where enough people are immune from the virus… to slow down its spread to the point that unprotected people have a natural buffer.