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.
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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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“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,” he added. “The estimates also suggest that we might have to recalibrate disease prediction models and rethink public health strategies.”
USC and county health officials plan to conduct further antibody tests every few weeks to continue to trace the disease. The antibody test is helpful for identifying past infection, but a polymerase chain reaction test is required to diagnose current infection, according to officials.
“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,” said Dr. Paul Simon, chief science officer at L.A. County Department of Public Health and co-lead on the study.
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The study’s results have not yet been peer reviewed by other scientists. The researchers plan to test new groups of participants every few weeks in coming months to gauge the pandemic’s trajectory in the region.
Researchers are testing for antibodies in adults over time in an effort to determine the scope and spread of the coronavirus pandemic across the county. The testing began in early April with a representative sample of 863 residents. A rapid antibody test was used for the study, which was conducted at six drive-through test sites.
City News Service contributed to this report.