An estimated 4.1% of Los Angeles County residents have been infected with novel coronavirus — dozens of times higher than officially confirmed cases, according to early results announced Monday by county health officials
The study, which is ongoing and has not been peer-reviewed, used an antibody blood test to estimate how many people had been infected with Covid-19 in the past. Other tests, like those performed with nasal swabs or saliva, test for the virus' genetic material, which does not persist long after recovery, as antibodies do.
The researchers, in collaboration with the University of Southern California, estimated that 221,000 to 442,000 adults in L.A. county had been infected based on drive-through tests conducted on April 10 and 11. Given the study’s margin of error, this represents between 2.8 and 5.6% of the population — which is also 28 to 55 times what county officials recorded around that time: about 8,000 confirmed cases.
"These results indicate that many persons may have been unknowingly infected and at risk of transmitting the virus to others," Dr. Barbara Ferrer, director of the L.A. County Department of Public Health, said in a statement Monday.
According to the announcement, new rounds of testing will be conducted "every few weeks” to follow the course of the outbreak at the local level over the coming months.
"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 the study’s lead investigator Neeraj Sood, a professor of public policy at USC Price School for Public Policy and senior fellow at USC Schaeffer Center for Health Policy and Economics. "The estimates also suggest that we might have to recalibrate disease prediction models and rethink public health strategies.”
On Friday, a similar study in Santa Clara County, California, estimated between 2.49% and 4.16% of residents had developed antibodies as of April 1. These numbers — representing between 48,000 and 81,000 people — were 50 to 85 times higher than the 956 confirmed cases at the time.
Similar efforts to estimate local antibody prevalence have launched in places like Miami-Dade County, Florida; San Miguel County, Colorado; and New York, New York. Experts say these studies could help improve projections and disease modeling, at the same time giving a more realistic sense of how deadly the virus really is.
Doctors suspect, though, and are still trying to prove beyond a doubt, that antibodies to Covid-19 mean one is immune down the line.