On April 26, 2022, this report was published online as MMWR Early release.
In December 2021, variant B.1.1.529 (Omicron) of SARS-CoV-2, the virus that causes COVID-19, became dominant in the United States. Subsequently, national cases of COVID-19 peaked at their highest recorded levels. * Traditional disease surveillance methods do not cover all cases of COVID-19, as some are asymptomatic, undiagnosed or unreported; therefore, the proportion of the population with antibodies to SARS-CoV-2 (ie, seroprevalence) may improve the understanding of the incidence of COVID-19 at the population level. This report uses data from the National Commercial Laboratory Study of CDC Seroprevalence and the 2018 Community Study to examine US trends in SARS-CoV-induced seroprevalence in September 2021 – February 2022, by age groups.
The National Trade Laboratory Seroprevalence Study is a multiple, cross-sectional national study that estimates the proportion of the population in 50 U.S. states, the District of Columbia and Puerto Rico that has infection-induced antibodies to SARS-CoV-2. † Serums tested for anti-nucleocapsid (anti-N) antibodies that are produced in response to infection but are not produced in response to vaccines against COVID-19 that are currently authorized for emergency use or approved by the Food and Drug Administration in the United States. §
In September 2021 – February 2022, a convenient sample of blood samples submitted for clinical examination was analyzed every 4 weeks for anti-N antibodies; in February 2022, the sampling period was <2 weeks in 18 of the 52 jurisdictions and no specimens were available from two jurisdictions. Samples for which the clinician has ordered a test for antibodies to SARS-CoV-2 are excluded to reduce selection bias. In September 2021 - January 2022, the average sample size for a 4-week period was 73,869 (range = 64,969-81,468); the sample size for February 2022 was 45,810. Seroprevalence estimates were assessed over 4-week periods as a whole and by age group (0–11, 12–17, 18–49, 50–64 and ≥65 years). To make estimates, the researchers weighed the results at the level of jurisdiction over the population, using measurements of age, gender and city status from data from the 2018 American Community Survey¶ (1). CIs were calculated using resampling (2); statistical differences were assessed by non-overlapping CI. All samples were tested by immunoglobulin immunoassay of Roche Elecsys Anti-SARS-CoV-2. ** All statistical analyzes were performed using R statistical software (version 4.0.3; The R Foundation). This activity was reviewed by the CDC, approved by the respective institutional review councils and conducted in accordance with applicable federal law and CDC policy.
In September-December 2021, the total seroprevalence increased by 0.9-1.9 percentage points over a 4-week period. In December 2021 – February 2022, the total seroprevalence in the United States increased from 33.5% (95% CI = 33.1-34.0) to 57.7% (95% CI = 57.1-) 58.3). During the same period, seroprevalence increased from 44.2% (95% CI = 42.8–45.8) to 75.2% (95% CI = 73.6–76.8) among children aged 0–11 years and of 45.6% (95% CI = 44.4–46.9). ) to 74.2% (95% CI = 72.8–75.5) among persons aged 12–17 years (Figure). Seroprevalence increased from 36.5% (95% CI = 35.7–37.4) to 63.7% (95% CI = 62.5–64.8) among adults aged 18–49, 28.8 % (95% CI = 27.9–29.8) to (459). % CI = 48.5–51.3) among those aged 50–64 and from 19.1% (95% CI = 18.4–19.8) to 33.2% (95% CI = 32, 2–34.3) among those aged ≥65 years.
The findings in this report are subject to at least four limitations. First, a convenient sample can limit generalizability. Second, the lack of data on race and ethnicity precludes weighting of these variables. Third, all samples were obtained for a clinical trial and may represent more people with greater access to health care or who seek medical care more often. Finally, these findings may underestimate the cumulative number of SARS-CoV-2 infections, as post-vaccination infections may lead to lower titers of anti-N, §§, ¶¶, and anti-N seroprevalence cannot explain re-infection.
As of February 2022, approximately 75% of children and adolescents had serological evidence of previous SARS-CoV-2 infection, and approximately one-third had become newly seropositive since December 2021. The largest increase in seroprevalence was in September 2021. – February 2022, the age groups with the lowest vaccination coverage; the proportion of the U.S. population fully vaccinated by April 2022 increases with age (5–11, 28%; 12–17, 59%; 18–49, 69%; 50–64, 80%; and ≥65 years) *** Lower seroprevalence among adults ≥65 years of age who are at higher risk of severe COVID-19 disease may also be associated with increased use of additional precautions with increasing of age (3).
These findings illustrate the high infection rate for the Omicron variant, especially among children. Seropositivity for anti-N antibodies should not be construed as protection against future infection. Vaccination remains the surest strategy to prevent complications from SARS-CoV-2 infection, including hospitalization in children and adults (4,5). Vaccination against COVID-19 after infection provides additional protection against serious illness and hospitalization (6). It is recommended that you keep up to date with vaccination for all eligible people, including those with a previous SARS-CoV-2 infection.
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