This research letter reports results from the first phase of a COVID-19 antibody study in Los Angeles County.

The authors tested for SARS-CoV-2–specific antibodies using a lateral flow immunoassay test among a random sample of Los Angeles County, CA residents between April 10 and April 14, 2020. They used these data to estimate the population prevalence of SARS-CoV-2 antibodies using weights to approximate a population-based sample.

They performed 865 tests among a sample that was 60% women, 55% aged 35 to 54, 58% white, and 43% high income earners (household incomes >$100,000 per year). 13% reported fever with cough, 9% fever with shortness of breath, and 6% loss of smell or taste.

Of this group, 35 individuals – 4.06% – tested positive. The weighted proportion of participants who tested positive was 4.65% (bootstrap CI, 2.52%-7.07%). This estimate implies that approximately 367 000 adults had SARS-CoV-2 antibodies, which is substantially greater than the 8430 cumulative number of confirmed infections in the county on April 10.