Prevalence of COVID-19 Antibodies Surges in Los Angeles County

Approximately 367,000 adults had the novel coronavirus 2019 (COVID-19) in Los Angeles County—a substantially greater count than the number of confirmed infections in the surrounding area, according to a research letter published in JAMA.

Investigators from California and Utah conducted serologic tests for COVID-19 in Los Angeles County in order to estimate cumulative incidence of the virus. Most COVID-19 reporting relies on PCR testing for infection confirmation, which can miss individuals who have recovered from infection, those that have mild or no symptoms, and those who have not been tested due to a limited number of tests available.

For their investigation, the investigators chose serologic testing as it can detect both active and past infections, they explained.

Residents of the county within a 15-mile radius of the testing site were eligible to participate, and residents could choose from 6 testing sites between April 10 -11, 2020. Those who wanted to participate, but were unable to come, were offered in-home testing on April 13-14.

Investigators used a proprietary database that represented the makeup of the county to select their participants. They invited a random sample, though only 1 participant per household was allowed. They also noted they calculated their results to match the 2018 census on sex, race/ethnicity, and income for Los Angeles County.

A total of 863 residents were included in the analysis, which was about half of those who provided consent to participate in the antibody testing, the investigators said. Of those, 60% were women, 55% were aged between 35 and 54 years, and 58% were white. Additionally, 43% had a yearly household income greater than $100,000, they found.

The patients reported symptoms such as fever with cough (13%), fever with shortness of breath (9%), and loss of smell or taste (6%), the study authors learned.

Just 35 participants (4.06%) tested positive, the study authors said, and that group varied by race/ethnicity, sex, and income. When the investigators calculated the weighted proportion of participants who tested positive, they concluded it was 4.31%. Then, they adjusted for test sensitivity and specificity, and found the rate of unweighted and weighted prevalence of COVID-19 antibodies was 4.34% and 4.65%, respectively.

From their calculations, the study authors estimate that approximately 367,000 adults have COVID-19 antibodies. That’s a number significantly greater than the 8,430 confirmed infections in the county as of April 10.

“Therefore, fatality rates based on confirmed cases may be higher than rates based on number of infections,” investigators wrote. “In addition, contact tracing methods to limit the spread of infection will face considerable challenges.”

However, they conceded limitations to their study has limitations; that selection bias is likely. They wrote that the estimated prevalence could potentially be biased due to nonresponse or symptomatic persons being more likely to participate.

They also said that as tests become more accurate, their estimates may change. They added that their study only focused on 1 county, so they said that serologic testing in other areas is warranted. This type of testing and monitoring could track the progress of the epidemic.