On May 19, 2020, a University of Southern California (USC) study reported results from the first phase of a COVID-19 antibody study in Los Angeles County have been published 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.

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.

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.