Study: Roughly 4% Of LA County Adults Have Had Coronavirus

Medical student volunteers from Keck School of Medicine of USC administer antibody tests to randomly selected participants as a part of the USC-L.A. County COVID-19 Study. (Kit Karzen)

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A study that tested 863 adults for antibodies to COVID-19 suggests it's infected more people in L.A. County than the official numbers show.

With the preliminary results released today, researchers estimate that somewhere between 220,000 and 440,000 Angelenos have already been infected. That's a lot more than the 13,816 cases confirmed so far.

With the substantially higher estimate of cases, that suggests the mortality rate for the county is much lower than the current 4.4%, said L.A. County Public Health Director Dr. Barbara Ferrer. She pegged it at closer to .1% or .2%.

"I think if there is a part of this that's hopeful for us, it is that the mortality rate now has dropped a lot," she said.


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The study suggests the vast majority of people in the county have probably not been infected.

"What the findings show is that only 4% of our population has been infected," said USC's Dr. Neeraj Sood, the study's lead investigator. "Which means we are very early in the epidemic and many more people in L.A. County could potentially be infected."

Noting that the sample size for the study was relatively small, Ferrer provided some early estimates of who was most likely to have been infected. Men, for instance, had a higher prevalence of infection than women (6% for men vs. 2% for women). As for demographics, she said the breakdown for those who tested positive was:

  • 7% of African Americans
  • 6% of white residents
  • 4.2% of Asians
  • 2.5% of Latino/Latina residents
Medical student volunteers from Keck School of Medicine of USC administer antibody tests to randomly selected participants as a part of the USC-L.A. County COVID-19 Study. (Kit Karzen)

L.A. County is currently averaging about 50 COVID-19 deaths a day, according to the Department of Public Health's Chief Science Officer, Dr. Paul Simon. He noted that COVID-19 is currently killing more people in the county than any other cause, including coronary heart disease.

And while an estimated mortality rate of .1% or .2% would get COVID-19 closer to the season flu's rate, researchers cautioned that's not the whole picture.

"For the flu, maybe deaths are spread over six to eight months," said lead investigator Dr. Neeraj Sood, professor & senior fellow at USC's Price School for Public Policy. "But for COVID, [they] might be more concentrated over a span of two months or one month, which could overrun the health care system — and that in turn could change the fatality rate."

Researchers hope to replicate the antibody study in the coming weeks.