Antibody study shows coronavirus infection more widespread in L.A. County than previously thought

May 18, 2020

(Photo/Kit Karzen)

The seroprevalence study indicates that despite higher than expected numbers of infections, L.A. County is still far away from reaching herd immunity or the end of the epidemic. 

Contact: Leigh Hopper, (310) 308-0405 or lhopper@usc.edu

Results from the first phase of a COVID-19 antibody study in Los Angeles County have been published today in JAMA, the Journal of the American Medical Association.

After testing a community sample of residents for antibodies to the new coronavirus, researchers with USC and the Los Angeles County Department of Public Health calculated that many more individuals in L.A. County had SARS-CoV-2 antibodies, a sign of a past infection, than previously thought. However, most people in the L.A. region have not been infected.

The researchers estimate that approximately 4.65% of the county’s adult population has contracted the new coronavirus. As with all statistical estimates, there is a range of uncertainty or margin of error. The analysis suggests that the fraction of adults in Los Angeles who contracted the novel coronavirus could range from as low as 2.5% to just over 7%. The results verify preliminary results that had been released last month.

“The number of confirmed COVID-19 cases is a poor proxy for the extent of infection in the community,” said lead investigator Neeraj Sood, a USC professor of public policy at the USC Price School for Public Policy and senior fellow at the USC Schaeffer Center for Health Policy and Economics. “We need to update models and forecasts based on new evidence. We’re still far away from herd immunity or the end of the epidemic. We need to look at a longer time horizon when evaluating policy decisions.”

Herd immunity can disrupt an infection’s spread. It occurs when a large percentage of people have developed antibodies. Experts believe that herd immunity to the coronavirus would occur when at least 60% of people have antibodies either through vaccination or past infection.

The study’s estimated infection rate also indicates that the fatality rate and the rate of hospitalizations are likely lower than rates based on official, confirmed case counts.

The study’s methods

To conduct the study, the researchers used a proprietary database maintained by LRW Group, a market research firm. A random sample of residents was invited via email and phone calls, with quotas for enrollment based on age, sex, race and ethnicity. Participation was restricted to one adult per household.

Over the course of four days, the researchers successfully tested 863 adults using an antibody test from Premier Biotech. Of the participants, 60% were women, 55% were aged 35 to 54 years old, 58% were white, and 43% had yearly household incomes greater than $100,000. Several participants reported that they had experienced symptoms of COVID-19 infection in the past 12 months: 13% reported that they had a fever with cough, 9% had a fever with shortness of breath, and 6% loss of smell or taste.

Researchers analyzed the results to account for the accuracy of the tests as well as the demographics of the participants.

Sood pointed out two important caveats: One, the results may have been skewed by an over-representation of people who experienced COVID-like symptoms in the past, potentially driving the number of positive tests up. Second, Sood emphasized that there’s not one infection rate for the state or for the entire country. Infection rates vary by geography, income, race, ethnicity and neighborhood.

“If you really want to understand what’s happening in your community, you need to do the study in your community,” he said.

In addition, the authors noted that prevalence estimates could change with new information on the accuracy of test kits used.

The study is ongoing. Researchers and the county plan to conduct additional rounds of testing.


In addition to Sood, other authors and institutions contributing to the study include study co-lead Paul Simon, chief science officer at L.A. County Department of Public Health; Peggy Ebner of the Keck School of Medicine of USC; Daniel Eichner of the Sports Medicine Research & Testing Laboratory; Jeffrey Reynolds of LRW Group; and Eran Bendavid and Jay Bhattacharya of Stanford University School of Medicine.

The study was supported with funding from USC Schwarzenegger Institute, USC Lusk Center, USC President’s Office, the Jedel Foundation, LRW Group, Soap Box Sample, Stanford COVID-19 Seroprevalence Studies fund, Foundation for Clean Competition, Partnership for Clean Competition, and several individual donors.

Image credit: Kit Karzen