September 10, 2021
More than 80% of Americans aged older than 16 years had COVID-19 antibodies in May 2021, with 20.2% of those being infection-induced antibodies, according to new research published in JAMA. The study, called the multistate assessment of SARS-CoV-2 seroprevalence in blood donors (MASS-BD), also found that the overall prevalence of infection-induced antibodies increased from 3.5% to 20.2% between July 2020 and May 2021.
Investigators from 17 blood collection organizations participated in the study, which was funded by the Centers for Disease Control and Prevention and led by Vitalant Research Institute. Investigators tested about 2,000 blood donation samples per month for different SARS-CoV-2 and weighted the results to estimate overall infection and vaccination rates. They also weighted the results by age group, sex and race/ethnicity. In total, they tested more than 1.5 million donation specimens from an area representing 74% of the United States population.
The findings demonstrate how infection-induced SARS-CoV-2 seroprevalence increased over time and varied by geographic region, age, race and ethnicity. At the conclusion of the study period (May 2021), infection-induced antibody seroprevalence was higher in the Midwest (23.5%) and South (22.2%) compared to the Northeast (19.3%) and West (16.7%). However, when combined with vaccine-induced antibodies, seroprevalence in the Northeast (87.2%) in July 2021 was higher than estimates for the other census regions. Combined seroprevalence was lowest in the South (80.2%).
People aged 16-29 years had the highest infection-induced seroprevalence estimates during the duration of the study, increasing from 5.5% in July 2020 to 27% in May 2021. Combined seroprevalence among this age group was 76.1% at the conclusion of the study period. Among those aged 65 years and older, combined seroprevalence was 92.2% in May 2021. Investigators noted that, despite having disproportionately higher COVID-19 mortality rates, persons aged 65 years and older consistently had the lowest infection-induced seroprevalence, consistent with other seroprevalence studies and case data.
Among racial and ethnic groups, infection-induced seroprevalence estimates in July 2020 were highest for Hispanic (6.8%) and non-Hispanic Black (6.6%) Americans. By the end of the study period, these rates increased to 30% and 21.1%, respectively. In July 2021, combined seroprevalence rates for Asian, white, Hispanic and Black Americans were 91%, 83.8%, 81.5% and 79.2%, respectively. According to the authors, this is likely because Asian and white persons received vaccinations at a greater rate than Hispanic and Black Americans.
Investigators emphasized that findings from a national sample of blood donors may not be representative of the entire U.S. population, despite weighting to adjust for demographic differences. They also noted that the data pre-date the widespread transmission of the Delta variant. However, they believe that the findings can supplement public health surveillance to identify groups potentially at higher risk of infection. The data may also inform infectious disease modeling and other research. Additional studies should evaluate the association between combined seroprevalence, protection and herd immunity.