An antibody study released Monday found that the novel coronavirus infection rate in Los Angeles County “far exceeds” the number of confirmed cases, meaning that the fatality rate is also “much lower” than previously thought.

The preliminary results of research conducted by USC and the Los Angeles County Department of Public Health found that about 4.1% of the county’s adult population carries the COVID-19 antibody, about 28 to 55 times higher than the 7,994 confirmed cases reported in early April.

Adjusted for the margin of error, the percentage of adults with the antibody ranges from 2.8% and 5.6%, which translates to between 221,000 and 442,000 adults. The county had recorded 617 deaths from the virus as of Monday.

“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 public health department, in a press release.

A similar study released last week by Stanford University showed a similar phenomenon in Santa Clara County, finding that 2.8% to 4.2% of residents tested carrying antibody resistance to the virus, a percentage that far exceeded the number of recorded cases.

“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 USC professor and lead investigator Neeraj Sood. “The estimates also suggest that we might have to recalibrate disease prediction models and rethink public health strategies.”

USC and the @CountyofLA are releasing preliminary results from the first round of antibody testing. The research is being led by @USCPrice’s Neeraj Sood, who will be answering questions: https://t.co/fKuGAG1lAG — USC (@USC) April 20, 2020

The first round of study results, which have not been peer-reviewed, came from a rapid antibody test administered April 10-11 at six sites in Los Angeles County, using 863 participants who were “recruited via a proprietary database that is representative of the county population.”

The antibody test “is helpful for identifying past infection, but a PCR [polymerase chain reaction] test is required to diagnose current infection,” said the department in a statement.

“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,” said Dr. Ferrer.

Interest in antibody testing has risen amid early indications that the infection may have spread much more broadly across the U.S. population than the number of positive cases, which would mean that the death rate is also far lower than suggested.

The Stanford study projected a mortality rate from the coronavirus of 0.12% to 0.2%. In early March,

the World Health Organization found that the mortality rate from COVID-19 worldwide was 3.4%, while the death rate from the seasonal flu is typically about 0.1%.

“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 the county health department and co-lead on the study.

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