LOS ANGELES, CA — The coronavirus likely swept through Los Angeles County, infecting hundreds of thousands of residents long before widespread testing was available, according to a new study released by USC and the Los Angeles County Department of Public Health.

That means the virus isn't as deadly as it seems, but the risk of infection is much higher than previously thought. As many as 221,000 to 442,000 adults were already infected by early-April, according to researchers, who conducted random antibody tests to estimate just how far the coronavirus has already spread in Los Angeles County. According to health officials, 2.8% to 5.6% of the county's adult population have antibodies to the virus. Based on those estimates, as many as 1 in 20 LA County adults have already been infected, most of whom hadn't been tested or hospitalized for COVID-19, the diseases caused by the coronavirus. That's 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 study indicates that the coronavirus likely spread earlier and further than previously thought and thousands more are likely infected now without knowing it. It also likely means the COVID-19 death rate is lower than it appears to be locally. As of Monday, the number of COVID-19 cases confirmed by testing stood at 13,816 — a fraction of the maximum estimate of 442,000 cases indicated by the antibody study. At least 617 people have died from the coronavirus in Los Angeles County. Don't miss local and statewide news about coronavirus developments and precautions. Sign up for Patch alerts and daily newsletters.



It remains to be seen if people have a degree of immunity to COVID-19 once they've already been infected as is the case with other viruses such as the common cold. "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. The numbers "reinforce the need" for people to continue engaging in social distancing, because there are many people in the county capable of spreading it to others who could become severely ill, added Ferrer.

The numbers prove that "we are very early in the epidemic," said lead investigator Neeraj Sood, a USC professor of public policy at USC Price School for Public Policy and senior fellow at USC Schaeffer Center for Health Policy and Economics.



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