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How Many People in Los Angeles County
Are Actually Infected by COVID-19?

Adults COVID-19 Infected, Los Angeles County

May 20, 2020 | Los Angeles Almanac

On April 20, 2020, the results of a first round of serology testing were released from a collaborative scientific study conducted by the University of Southern California and Los Angeles County Department of Public Health (Public Health). The study’s purpose is to determine the percentage of test populations that have antibodies as a result of having been infected with COVID-19 at some point and estimate the percentage of Los Angeles County’s population as a whole that might have been infected. The first results suggested that COVID-19 infections are much more widespread (and the fatality rate lower) in Los Angeles County than previously thought.

From the results of the first round of testing of about 863 adults, the research team estimated that approximately 4.1% of Los Angeles County's adult population (or about 1 in 25 adults) possess antibodies to COVID-19 (with a statistical margin of error that ranges from about 2.8% to 5.6% of the adult population). This indicates that about 254,000 to 509,000 Los Angeles County adults have been infected by the virus at some point up until the time of the study in early April. This estimate suggests that the number of persons in the county who actually have been infected by COVID-19 is 28 to 55 times higher than the number of confirmed cases of COVID-19 currently reported to the county.

On May 20, 2020, results from a second round of testing of 1,014 different individuals were released, with about 2.1% having COVID-19 antibodies. The second round of testing, conducted in both the Spanish and Mandarin languages, was conducted at two different test sites than in the first round, in order to target different demographics. An explanation for the difference in percentages of those testing positive for the antibodies, between the two rounds, could be due to differences in the demographics tested, but could only be speculated at the moment. The study will continue to conduct additional tests.

"These results indicate that many persons may have been unknowingly infected and are 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 (PCR) testing to diagnose those with infection so they can be isolated and quarantined, while also maintaining the broad social distancing interventions."

For more information, see the Los Angeles County Department of Public Health release.