220,000 residents.. Study says way more people have been infected

Los Angeles County antibody study suggests over 220,000 residents have already had coronavirus

By Eric Ting

A few days after preliminary results from a large-scale antibody study in Santa Clara County suggested coronavirus infections in the county are underreported by a factor as large as 50 to 85, results from a newly-released antibody study conducted in Los Angeles County contained similar findings.

The L.A. antibody study was conducted by the University of Southern California and the Los Angeles County Department of Public Health on April 10 and 11, and is part of an ongoing study where 1,000 residents will be tested each week. Unlike the Stanford study, where participants were recruited via Facebook ads, participants in the Los Angeles County were recruited by market services firm LRW Group, which used a large proprietary database to create a random sample of the county population.

Of the 1,000 individuals tested in early April, 4.1 percent were found to have COVID-19 antibodies. When adjusting for statistical margin of error, the study finds that 2.8 percent to 5.6 percent of the county's adult population has already been infected, which translates to 221,000 to 442,000 people. There were nearly 8,000 confirmed cases when the study was conducted, which leads researchers to believe the county is underreporting infections by a factor of 28 to 55.

The Santa Clara study found a 1.5 percent prevalence of antibodies, a figure researchers scaled up to 2.5 percent to 4.2 percent when adjusting for population and test performance characteristics. The weighted figures suggest underreporting by a factor of 50 to 85, whereas the unweighted figure suggests underreporting by a factor of 30.

If infections are this vastly underreported, then the mortality rate of COVID-19 is substantially lower than current estimates.

There was no official adjusted mortality rate reported in the Los Angeles County study. Simply dividing the number of deaths on April 11 by the number of infections is not an accurate metric, as there is a significant lag time between infection and death. The Stanford researchers, who conducted their study on April 3 and April 4, projected deaths through April 22 and divided that figure by the number of infections to calculate a "true" mortality rate of .12 to .20 percent when using the weighted figures.

However, a number of outside researchers cautioned against using the weighted estimates and recommended using the unweighted figure instead. When using the unweighted numbers and the study's lagged death projections, one gets a "true" mortality rate of .35 percent, a number almost identical to the mortality rate calculated following antibody tests in a hard-hit German town.

In order to gauge the "true" mortality rate in Los Angeles County, one would need a projection for deaths through April 29 to match the 19-day lag the Stanford researchers used when making their new mortality rate calculations.

"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 lead investigator Neeraj Sood, a USC 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."

County health officials stated that despite the lower mortality rate, the fact that only four percent of the population has been infected highlights the need for control efforts, whatever those may be.

"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 co-lead investigator Dr. Paul Simon, chief science officer at L.A. County Department of Public Health.