LOS ANGELES – Preliminary results released Monday from an antibody testing program indicates the number of COVID-19 infections in Los Angeles County far exceeds the number of confirmed case announced to date, with potentially more than 400,000 residents having been infected at some point.
Researchers said the good news from the antibody study conducted by USC and the county is that the mortality rate from the illness is much lower than currently estimated. The bad news is that there are potentially hundreds of thousands of residents who are unknowingly infected with the illness and may not be showing any symptoms, but are still capable of spreading it to others.
“It does suggest that the number of total people in the county with a past or current infection is 28 to 55 times higher than the number of reported positive cases,” county public health director Barbara Ferrer said. As of Monday, the number of reported positive cases in the county was just more than 13,800.
She said the antibody serology tests show that with thousands of residents likely infected and capable of transmitting the virus, social-distancing requirements need to remain in place.
“It does for me reinforce the need for everyone to continue to stay home, to observe all of our physical-distancing requirements because there are many, many people throughout the county who are positive for COVID-19 who may not be showing any symptoms.
“… This means that given the high rate of people that may have been infected at some point with COVID-19, we need to assume 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,” she said. “That means keeping our distance, using our cloth face coverings when we’re in close contact with people and staying home if you’re sick so you don’t expose anybody else to any of your germs.”
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. The researchers plan to test new groups of participants every few weeks to gauge the pandemic’s trajectory.
Based on the first round of testing, the research team estimated that roughly 4.1% of the county’s adult population has developed antibodies to the virus — an indication they are or were infected at some point. Adjusting for the statistical margin of error of the test implies that approximately 221,000 to 442,000 adults in the county have had the infection.
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.
“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 L.A. County could potentially be infected,” said professor Neeraj Sood of the USC Price School for Public Policy and the lead investigators on the study. “And as those number of infections rise, so will the number of deaths, the number of hospitalizations and the number of ICE admissions.
“The study says we need to continue monitoring the epidemic through serology testing and other means,” he said. “We need to take the data we are getting and update and re-calibrate our models of what will happen in the future as more people get infected, and then ultimately use this better model as the more accurate data to guide public policy decisions about how to combat this epidemic.”
Sood noted that the only people tested in the county to date have been people with symptoms, and that testing has been limited, while the antibody tests paint a far better picture of how widespread the infections are.
The results have important implications for public health efforts to control the local epidemic.
“These results indicate that many persons may have been unknowingly infected and at risk of transmitting the virus to others,” Ferrer said. “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.”
The antibody test is helpful for identifying past infection, but a PCR test is required to diagnose current infection.
More than 600 people have died of the virus countywide.
“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, who co-led the study.
The study’s results have not yet been peer reviewed by other scientists. The test’s accuracy was assessed at a lab at Stanford University, using blood samples that were positive and negative for COVID-19.
The study was supported with funding from the USC Schwarzenegger Institute, USC Lusk Center, USC President’s Office, Jedel Foundation, LRW Group, Soap Box Sample, and several individual donors.