LA County study finds COVID-19 ‘herd immunity’ unlikely

In a research letter published in the Journal of the American Medical Association (JAMA), researchers conducted a study of seroprevalence and vaccination coverage in Los Angeles County (LAC).

Immunity to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) can be obtained by vaccination or by previous infection. This potential protective immunity should exist at the population level to effectively mitigate the coronavirus disease 2019 (COVID-19) pandemic. However, various reports of antibodies (Abs) against SARS-CoV-2 decreasing after a certain period have been documented, and therefore, it is essential to monitor the seroprevalence of these Abs and other cell-mediated immune responses under the adaptive arm of the immune system.

Research Letter: Seroprevalence of SARS-CoV-2 Receptor Binding Domain-Specific Antibodies and Vaccination Coverage in Los Angeles County Adults, April 2021: Pandemic LA Surveillance Cohort Study. Image Credit: ktsdesign/Shutterstock

The study

In the present study, researchers investigated the presence of Abs specifically in the receptor binding domain (RBD) of the SARS-CoV-2 spike (S) protein following partial or full vaccination or from a natural infection. Abs versus RBD (RBD Abs) were assessed by the Luminex xMAP SARS-CoV-2 assay. This is a cross-sectional study conducted at eight testing sites in LAC between April 9 and April 25, 2021, on residents living within a 15-mile radius. Participants were randomly selected and invited for testing using a proprietary database.


The initial screening selected approximately 5,500 eligible adults, and approximately 2,314 (42%) of these had given consent for testing. Among them, antibody tests were carried out on 1,335 individuals. Women made up 59.2% of the participants tested, just over 50% of them were between the ages of 30 and 49, 13.9% were Asian and 9.4% were black. About 30% of the study population had an annual family income of less than $50,000.

The authors observed that the black population (52.5%) and people from low-income households (61.2%) had lower protection rates. Only 28.8% of unvaccinated people with a history of infection had specific antibodies to S RBD, and the seroprevalence rate was observed to be relatively higher in poorer regions (71%). RBD Abs in the fully vaccinated population remained consistent, with more than 99.7% of them demonstrating the presence of RBD Abs.


The findings of the present study point to the existence of potential protective immunity or Abs against RBD SARS-CoV-2 in over 72% of LAC residents.

Despite the high seroprevalence of LAC, an increase in COVID-19 cases was reported in July 2021, indicating the difficulty of achieving herd immunity. Blacks and poorer people have shown a disparity in vaccination rates, and increased efforts are being made to target these groups.

Another important finding is the high percentage of Abs RBD in the unvaccinated adult population of high poverty areas. Antibodies were observed in all who had documented COVID-19 in the past, even after several months of infection, suggesting that RBD Abs caused by SARS-CoV-2 infection are not decreasing. This study included self-reported vaccinations, and the presence of Abs as markers of immunity instead of assessing cell-mediated immunity and selection bias might limit the validity of the observations made.

Journal reference:

  • Sood, Neeraj, Olivier Pernet, Chun Nok Lam, Angela Klipp, Rani Kotha, Andrea Kovacs and Howard Hu. “SARS-CoV-2 receptor-binding domain-specific antibody seroprevalence and vaccination coverage among adults in Los Angeles County, April 2021: LA Pandemic Surveillance Cohort Study.” Open JAMA Network 5, no. 1 (2022 Jan 20): e2144258, DOI:,

Comments are closed.