Estimated SARS-CoV-2 Seroprevalence Among Persons Aged <18 Years — Mississippi, May–September 2020

As of March 1, 2021, persons aged <18 years accounted for approximately 11% of 28.4 million reported COVID-19 cases in the United States*; however, data on pediatric infections with SARS-CoV-2, the virus that causes COVID-19, are limited (1). Surveys of SARS-CoV-2 antibody seroprevalence suggest that cumulative incidence of infection is much higher than that ascertained by reported COVID-19 cases (2,3). Evidence of previous SARS-CoV-2 infections among young persons in Mississippi was assessed by testing for antibodies to SARS-CoV-2 on a convenience sample of residual serum specimens collected for routine testing by an academic medical center laboratory during May 17–September 19, 2020. Seroprevalence by calendar month was standardized to the state population by race/ethnicity; cumulative numbers of infections were estimated by extrapolating seroprevalence to all persons aged <18 years in Mississippi. Serum specimens from 1,603 persons were tested; 175 (10.9%) were positive for SARS-CoV-2 antibodies. Among 1,579 (98.5%) specimens for which the race/ethnicity of the person tested was known, specimens from 16 (23.2%) of 69 Hispanic persons, 117 (13.0%) of 901 non-Hispanic Black persons, and 30 (5.3%) of 565 non-Hispanic White persons tested positive. Population-weighted seroprevalence estimates among persons aged <18 years increased from 2.5% in May to 16.3% in September 2020. Based on these estimates, 113,842 (95% confidence interval [CI] = 90,096–153,652) persons aged <18 years in Mississippi might have been infected with SARS-CoV-2 by mid-September 2020. The number of COVID-19 cases reported in this age group through August 31, 2020 was 8,993. Serosurveys that include pediatric age groups can help provide evidence of cumulative disease incidence, estimate frequency of undiagnosed cases of SARS-CoV-2 among young persons, and guide prevention efforts.

Most persons who are infected with SARS-CoV-2 develop antibodies to SARS-CoV-2 proteins within 1–2 weeks of disease onset (4). Serologic testing for SARS-CoV-2 antibodies, albeit having imperfect sensitivity and specificity,^† is useful to identify past SARS-CoV-2 infections. Serology tests are used widely in seroprevalence studies to understand patterns of virus spread and cumulative incidence of SARS-CoV-2 infection^§ (2,3).

This retrospective seroprevalence study was conducted by the University of Mississippi Medical Center in collaboration with the Mississippi State Department of Health (MSDH) and CDC to describe trends in SARS-CoV-2 antibody seroprevalence among young persons in Mississippi during the COVID-19 pandemic. The University of Mississippi Medical Center provides clinical laboratory services for university hospitals in central Mississippi and 12 hospitals outside the university network statewide (5). Demographic data including age, sex, race/ethnicity, and date of collection were obtained for deidentified residual serum specimens collected for routine clinical testing during May 17–September 19, 2020, from persons aged <18 years. One specimen per person was included in the analysis, either the first seropositive specimen or the earliest specimen from persons with all seronegative specimens, to avoid potential bias in underestimating infections from decline in antibodies below the limit of detection for seropositivity. Sera were stored at −20°C (−4°F) before testing at CDC.

Seropositivity was determined for serum specimens using one of two assays, based on specimen volume. Specimens with adequate volume (≥0.3 mL) were tested with a qualitative VITROS anti–SARS-CoV-2 total antibody in vitro diagnostic test using the automated VITROS 3600 Immunodiagnostic System (Ortho Clinical Diagnostics) (6). One aliquot was heat-treated at 56°C (132.8°F) for 10 minutes and tested on the VITROS Immunodiagnostic System. An automatically calculated ratio of test sample signal to cutoff value (S/C) <1.0 was interpreted as nonreactive, and S/C ≥1.0 was interpreted as reactive for anti–SARS-CoV-2 total antibody (6). Samples with volumes <0.3 mL were tested to determine seropositivity using an enzyme linked immunosorbent assay (ELISA) developed by CDC to measure total SARS-CoV-2 antibodies against the extracellular domain of the SARS-CoV-2 spike protein (2).^¶

Seroprevalence by calendar month was standardized to the Mississippi population aged <18 years by race/ethnicity**; 95% CIs accounting for assay test performance were estimated by using published methods (2). Cumulative numbers of SARS-CoV-2 infections were estimated by extrapolating seroprevalence and 95% CIs to the Mississippi population aged <18 years and were compared with cumulative numbers of confirmed and probable COVID-19 cases (as defined by the Council of State and Territorial Epidemiologists)^†† in persons aged <18 years reported to MSDH.^§§ Ratios of estimated SARS-CoV-2 infections to reported COVID-19 cases were calculated by dividing estimated numbers of SARS-CoV-2 infections by the reported cumulative number of COVID-19 cases as of the last day of the preceding month. Statistical analyses were conducted using SAS (version 9.4; SAS Institute). This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.^¶¶ The project was also reviewed and approved by the University of Mississippi Medical Center Institutional Review Board through its expedited review procedure.

Among 1,603 serum specimens from persons aged <18 years included in analyses, 175 (10.9%) tested positive for SARS-CoV-2 antibodies, including 152 of 1,469 (10.4%) by VITROS assay and 23 of 134 (17.2%) by ELISA (Table 1). Among 1,579 (98.5%) specimens for which the race/ethnicity of the person receiving testing was known, specimens from 16 (23.2%) of 69 Hispanic persons, 117 (13.0%) of 901 non-Hispanic Black persons, and 30 (5.3%) of 565 non-Hispanic White persons tested seropositive. After adjusting by race/ethnicity to the Mississippi population aged <18 years, estimated seroprevalence increased from 2.5% in May to 16.3% in September (Figure). Extrapolating to the state population, an estimated 113,842 (95% CI = 90,096–153,652) Mississippi residents aged <18 years might have been infected with SARS-CoV-2 by mid-September 2020; through August 31, a total of 8,993 COVID-19 cases among persons aged <18 years had been reported to MSDH (Table 2). Ratios of estimated numbers of SARS-CoV-2 cases based on seropositivity to COVID-19 cases reported at the end of the preceding month decreased from 68.2 in May to 12.7 in September. This finding suggests an improvement in case detection over time, even though the number of SARS-CoV-2 cases estimated from seroprevalence was consistently higher than the number of SARS-CoV-2 cases reported during each month.

Acknowledgments

Lora M. Martin, Children’s of Mississippi, Department of Pediatrics, Division of Infectious Disease, University of Mississippi Medical Center, Jackson, Mississippi; Brian Kirmse, Department of Pediatrics, Division of Genetics, University of Mississippi Medical Center, Jackson, Mississippi; Eugene Melvin, Center for Informatics, University of Mississippi Medical Center, Jackson, Mississippi; CDC COVID-19 Task Force Clinical Laboratory Improvement Amendments Diagnostic Reference Team; CDC COVID-19 Task Force Microbial Pathogenesis and Immune Response Laboratory.

Corresponding author: Charlotte V. Hobbs, chobbs@umc.edu.

¹Children’s of Mississippi, University of Mississippi Medical Center, Jackson, Mississippi; ²CDC COVID-19 Response Team; ³Mississippi State Department of Health.

References

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Abbreviations: CI = confidence interval; ELISA = enzyme-linked immunosorbent assay.
* Includes residual serum specimens from 1,603 persons aged <18 years.
^† 95% CIs for seroprevalence by month were calculated assuming a binomial distribution for SARS-CoV-2 seropositivity in the sample.
^§ P-value for difference in proportion seropositive by individual characteristic, serologic assay, or dates of specimen collection using chi-squared text.
^¶ P-value for difference in proportion seropositive in specimens tested by each assay; individual specimens were tested with Ortho VITROS if specimen volume was ≥0.3 mL or CDC ELISA if specimen volume was <0.3 mL.

FIGURE. Cumulative number of reported COVID-19 cases and estimated race/ethnicity–standardized SARS-CoV-2 antibody seroprevalence* among persons aged <18 years — Mississippi, April–September 2020

Abbreviation: CI = confidence interval.

* From residual serum specimens collected during May 17–September 19, 2020, from persons aged <18 years.

Abbreviation: CI = confidence interval.
* Standardized to the race/ethnicity distribution of the Mississippi population aged <18 years using weights derived from the 2019 U.S. Census; 95% CIs were calculated for race/ethnicity–weighted SARS-CoV-2 seropositivity in the sample accounting for assay test performance.
^† Race/ethnicity–standardized seroprevalence estimates for each period applied to the 2019 Mississippi population aged <18 years = 698,420 (https://www.census.gov).
^§ Cumulative number of confirmed and probable COVID-19 cases in persons aged <18 years reported to Mississippi State Department of Health as of the end of the preceding month.