USC and the Los Angeles County Department of Public Health released preliminary results on April 20 from a collaborative scientific study that suggests COVID-19 infections are far more widespread – and the fatality rate much lower – than previously thought in Los Angeles County.

The results are from the first round of an ongoing study by USC researchers and county public health officials. They will be conducting antibody testing over time on representative samples of adults countywide to determine the scope and spread of the pandemic across the county.

Based on results of the first round of testing, the research team estimates that approximately 4.1% of the county’s adult population has antibodies to the virus. Adjusting this estimate for statistical margin of error implies 2.8% to 5.6% of the county’s adult population has antibodies to the virus, which translates to approximately 221,000 to 442,000 adults who have had the infection. That estimate is 28 to 55 times higher than the 7,994 confirmed cases of COVID-19 reported to the county by the time of the study in early April. The number of COVID-19 deaths countywide was 729 as of April 22.

“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, a professor of public policy at USC Price School for Public Policy and senior fellow at USC Schaeffer Center for Health Policy and Economics. “The estimates also suggest that we might have to recalibrate disease prediction models and rethink public health strategies.”

The results have important implications for public health efforts to control the epidemic, public health authorities said.

“These results indicate that many persons may have been unknowingly infected and at risk of transmitting the virus to others,” said Dr. Barbara Ferrer, director of the L.A. County Department of Public Health. “These findings underscore the importance of expanded polymerase chain reaction testing to diagnose those with infection so they can be isolated and quarantined, while also maintaining the broad social distancing interventions.”

The antibody test is helpful for identifying past infection, but a PCR test is required to diagnose current infection. For information, visit lacounty.gov.