What was the SARS-CoV-2 seroprevalence among US adults during 2021 and 2022?

By Neha MathurDec 7 2022Reviewed by Aimee Molineux

In a recent Morbidity and Mortality Weekly Report (MMWR), the United States Center for Disease Control and Prevention (U.S.-CDC) published the preliminary results of the National Health and Nutrition Examination Survey (NHANES) for August 2021–May 2022.

The CDC has been monitoring severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroprevalence in the US public since the coronavirus disease 2019 (COVID-19) pandemic began. They pursue data for asymptomatic cases of COVID-19 that likely remain undiagnosed and unreported.

The nonrandom sampling of data from blood donors from commercial diagnostic laboratories often affects SARS-CoV-2 serological estimates. On the contrary, NHANES uses a two-year statistical sampling design to collect SARS-CoV-2 serology and self-reported COVID-19 history and vaccination data from the general U.S. population.

Background

Antibodies to SARS-CoV-2 spike (S) and nucleocapsid (N) proteins indicate previous infection or vaccination and previous infection only, respectively. However, data on what percentage of US adults have acquired anti-SARS-CoV-2 antibodies through vaccination, prior infection (or both) is missing.

About the study

In the present study, researchers quantified the SARS-CoV-2 seropositivity in the general U.S. population using NHANES data collected between August 2021 and May 2022.

Though the team sequentially visited data from 15 primary sampling units collected during the one-year data collection period, they could analyze early data for ≥ 18-year-old adults from the first ten units. This data corresponded to two COVID-19 waves due to the SARS-CoV-2 Delta and Omicron variants of concern (VOCs) between August and November 2021, as well as December 2021 and May 2022, respectively.

The researchers examined SARS-CoV-2 seroprevalence stratified by demographic characteristics, including age, sex, race, Hispanic origin, and education. Plus, they considered self-reported COVID-19 and vaccination receipts (one or more vaccine doses). They also estimated seroprevalence in individuals with combined anti-N–negative and anti-S–positive (vaccinated, but not infected) and anti-S–positive and anti-N–positive test results (infected, likely vaccinated).

Study findings

The NHANES serologic testing dataset had 1,574 participants aged ≥18 years during the two-year study period. In the study cohort, 91.5% and 41.6% of adults had SARS-CoV-2 anti-S and anti-N antibodies, respectively. The proportion of adults with documented anti-S–positive and anti-N–positive serology was 41.6% overall and diminished with age. Thus, it was 59.7% among younger 18 to 29-year-olds but merely 30.2% among those ≥70 years.

The percentage of vaccinated but not infected US adults was 49.9% overall. This percentage increased more in adults ≥70-year-olds than 18 to 29-year-olds, i.e., from 28.1% to 64.7%. However, it remained consistently lower among Hispanic and Black individuals and those who did not attend a high school, at 35.3%, 46.7%, and 42.5%, respectively. However, this percentage was also higher in White adults and those with college-level education, at 58.9% and 55.4%, respectively.

Though serology results of 655 study participants indicated infection, 43.7% reported they never contracted COVID-19, suggesting asymptomatic infection. Such self-reports were primarily from Black adults and those who did not attend a high school.

Among the adults infected but possibly vaccinated, 25.5% self-reported that they did not take a COVID-19 shot. Despite serologic evidence of infection, 31.3% of Black adults and 21.4% of Hispanic adults self-reported non-receipt of COVID-19 vaccination. Clearly, these ethnic groups acquired anti-S–positive and anti-N–positive antibodies through infection rather than vaccination.

Conclusions

To summarize, serologic testing in US adults found that ~42% had SARS-CoV-2 antibodies likely due to SARS-CoV-2 exposure,  but nearly 44% self-reported they never contracted COVID-19. Limited access to COVID-19 testing likely amplified undiagnosed SARS-CoV-2 infections among younger and Black adults in the US. It resulted in disparities in community transmission, infection rates, and outcomes. Overall, these results raise public health equity concerns.

Based on the survey results, the CDC recommends that all Americans get COVID-19 vaccinated. Moreover, they maintained that efforts to attain equity in primary series vaccination and booster dose coverage should continue.