SARS-CoV-2 Seroprevalence Study
Since July 2020, SARS-CoV-2 seroprevalence in the United States has been estimated by testing blood donors. CDC, in collaboration with Vitalant, American Red Cross, Creative Testing Solutions, and Westat, established a nationwide cohort of 142,758 blood donors in July 2021. The cohort included persons who had donated blood two or more times in the preceding year. All blood donations collected during April–June 2021 were tested for antibodies against the spike (S) and nucleocapsid (N) proteins. Beginning in 2022, up to one blood donation sample per donor was randomly selected each quarter and tested using the Ortho VITROS SARS-CoV-2 Quantitative S IgG and total N antibody tests. Both SARS-CoV-2 infection and COVID-19 vaccination result in production of anti-S antibodies but only infection produces anti-N antibodies.
Data analysis was limited to 72,748 donors for whom it was possible to ascertain immune status during each period using their prior classification of previously infected or vaccinated, antibody testing results, and their vaccination status at the time of each donation. During the first quarter examined (April–June 2021), an estimated 68.4% of persons aged ≥16 years had SARS-CoV-2 antibodies from previous infection or vaccination, including 47.5% from vaccination alone, 12.0% from infection alone, and 8.9% from both.
During January–March 2022, 93.5% of persons aged ≥16 years had antibodies from previous infection or vaccination, including 39.0% from vaccination alone, 20.5% from infection alone, and 34.1% from both.
During July–September 2022, 96.4% of persons had antibodies from previous infection or vaccination, including 26.1% with vaccine-induced immunity alone, 22.6% with infection-induced immunity alone, and 47.7% with hybrid immunity. During July–September 2022, the prevalence of infection-induced immunity was 85.7 among unvaccinated persons and 64.3% among vaccinated persons.
During July–September 2022, the lowest prevalence of hybrid immunity was observed in persons aged ≥65 years, and the highest, 59.6% in adolescents and young adults aged 16–29 years. During all periods, higher prevalences of hybrid immunity were observed among Black and Hispanic populations than among White and Asian populations.
|
Percent of Donors Positive for SARS-CoV-2 Antibodies |
|||
|
Antibodies from: |
Apr-Jun 2021 |
Jan-Mar 2022 |
Jul-Sep 2022 |
|
Infection |
12.0 |
20.5 |
22.6 |
|
Vaccination |
47.5 |
39.0 |
21.0 |
|
Hybrid Immunity |
8.9 |
34.1 |
47.7 |
|
Total |
68.4 |
93.5 |
96.4 |
Both infection-induced and hybrid immunity increased during the study period. More than 96% of people aged 16 years or older in the US had SARS-CoV-2 antibodies by September 2022. Approximately two thirds had been infected with SARS-CoV-2 and almost one half (47.7%) had developed hybrid immunity from both vaccination and infection. The increase in seroprevalence, including hybrid immunity, is likely contributing to lower rates of severe disease and death from COVID-19 in 2022–2023 than during the early pandemic.
The prevalence of hybrid immunity is lowest in adults aged ≥65 years, likely due to higher vaccination rates, earlier availability of COVID-19 vaccines, and closer adherence to public health measures to avoid infection. However, going forward the lower prevalences of infection-induced and hybrid immunity in this age group could increase the risk for severe disease, emphasizing the importance remaining current with COVID-19 vaccination.
Reference
Jones, Estimates of SARS-CoV-2 Seroprevalence and Incidence of Primary SARS-CoV-2 Infections Among Blood Donors, by COVID-19 Vaccination Status - United States. April 2021-September 2022. MMWR June 3, 2023;72:601-605.
