At least 200,000 people are believed to have been infected by the coronavirus outbreak in Los Angeles County by early April, according to a report on Monday. This figure would far surpass the number of officially confirmed cases in the United States.

The revelation was made in a large scale study by the University of Southern California and the Los Angeles County Department of Public Health, which found that 4.1 percent of the county’s adult population has antibodies to the virus in their blood, which is an indicator of past exposure. This means that between 221,000 and 442,000 adults in the area have previously been infected.

“We haven’t known the true extent of COVID-19 infections in our community because we have only tested people with symptoms, and the availability of tests has been limited,” said lead investigator Neeraj Sood, professor of public policy at the USC Price School for Public Policy. “The estimates also suggest that we might have to recalibrate disease prediction models and rethink public health strategies.”

According to the Los Angeles Times, which cited researchers, the results were determined from antibody testing of about 863 people who were representative of L.A. County. At the time of the study in early April, L.A. County reported 7,994 confirmed cases. However, the estimated numbers following the study are 28 to 55 times higher than the confirmed COVID-19 cases reported.

Barbara Ferrer, director of the L.A. County Department of Public Health, said the results show that a large percentage of the population unknowingly had the virus and are "at risk of transmitting the virus to others."

Paul Simon, chief science officer at the L.A. County Department of Public Health and co-lead on the study, said: “Though the results indicate a lower risk of death among those with infection than was previously thought, the number of COVID-related deaths each day continues to mount, highlighting the need for continued vigorous prevention and control efforts."

At least 615 coronavirus deaths have been confirmed in the county as of Tuesday morning, according to data from Johns Hopkins.

The number of confirmed cases of COVID-19 continues to grow in the U.S. Mike Pence, the vice-president, is overseeing the U.S. response to the coronavirus.