A new study suggests about 4% of Los Angeles County adults could have already had COVID-19 in early April. 

USC and the Los Angeles County Department of Public Health released preliminary results from a collaborative study Monday that traced the presence of COVID-19 antibodies in a small sample of the population. Of the adults in that study, 4.1%were found to have the antibodies, suggesting they had already been infected. 

“Adjusting this estimate for statistical margin of error implies about 2.8% to 5.6% of the county’s adult population has antibody to the virus, which translates to approximately 221,000 to 442,000 adults in the county who have had the infection,” the department said in a press release. “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-related deaths in the county has now surpassed 600.”

Dr. Neeraj Sood, a professor at USC Price School for Public Policy and one of the study authors, said the implication is that current estimates will need to be revised to account for the new data. 

“It says that mortality rates estimated based on confirmed cases are going to be much higher than mortality rates estimated based on the number of infections or the estimated number of infections,” he said. “So we could say that maybe the good news is the fatality rate is lower than what we thought it would be, but the fatality rate is not the only number we should focus on. That is not the only number that determines the burden of disease. What the findings show is that only 4% of our population has been infected, which means we are very early in the epidemic and many more people in Los Angeles County could potentially be infected and as those numbers of infections rise, so will the number of deaths, the number of hospitalizations and the number of ICU admissions.”

Public Health Director Dr. Barbara Ferrer said the figures reinforce the need to observe stay-at-home orders and maintain physical distancing because many more people throughout the county could be positive without showing symptoms. 

“I’ve talked before about the need to observe universal precautions,” she said. “This means that given the high rate of people that may have been infected at some point, we need to assume that at any point in time we could be infected and that all of the other people we come in contact with could also be infected and that means keeping our distance, using our cloth face coverings when we are in close contact with people and staying home if you are sick so that you don’t expose anybody else to any of your germs.”

Participants in the antibody study were recruited via a proprietary database that is representative of the county population. The database is maintained by LRW Group, a market research firm. The researchers used a rapid antibody test for the study. The FDA allows such tests for public health surveillance to gain greater clarity on actual infection rates. The test’s accuracy was further assessed at a lab at Stanford University, using blood samples that were positive and negative for COVID-19.

However, the study’s results have not yet been peer reviewed and the authors said they will continue to test new participations on an ongoing basis to track the spread of the infection. 

“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 Department of Public Health and co-lead on the study.



Matthew Hall

Matthew Hall has a Masters Degree in International Journalism from City University in London and has been Editor-in-Chief of SMDP since 2014. Prior to working at SMDP he managed a chain of weekly papers...

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