The true number of coronavirus cases in the U.S. could be anywhere from six to 24 times higher than the confirmed number of cases, depending on location, according to a large federal study that relied on data from 10 U.S. cities and states.
The vast majority of Americans, however, are still vulnerable to Covid-19.
The study, published Tuesday in JAMA Internal Medicine, 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.
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.
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 (which scientists expect will happen for some amount of time after an initial infection) to slow down its spread to the point that unprotected people have a natural buffer.
“The study rebukes the idea that current population-wide levels of acquired immunity (so-called herd immunity) will pose any substantial impediment to the propagation of SARS-CoV-2 in the U.S., at least for now,” infectious disease experts Tyler Brown and Rochelle Walensky of Massachusetts General Hospital wrote in an editorialaccompanying the study. Experts estimate that 60% to 70% of people in a given area would need to be protected from the virus — either through recovering from an infection or vaccination — to reach herd immunity.
