In December 2021, the B.1.1.529 (Omicron) variant of SARS-CoV-2, the virus that causes COVID-19, became predominant in the United States. Subsequently, national COVID-19 case rates reached their highest point on record. Traditional disease surveillance methods do not capture all COVID-19 cases because some are asymptomatic, undiagnosed, or unreported; therefore, the proportion of the population with SARS-CoV-2 antibodies (i.e., seroprevalence) may improve understanding of COVID-19 incidence at the population level.
This report uses data from the CDC National Commercial Laboratory Seroprevalence Study and the 2018 American Community Survey to examine US trends in infection-induced SARS-CoV-2 seroprevalence during September 2021. as of February 2022, by age group.
The National Commercial Laboratory Seroprevalence Study is a repeated cross-sectional national survey that estimates the proportion of the population in 50 US states, the District of Columbia, and Puerto Rico that has infection-induced antibodies to SARS-CoV-2 . Sera are tested for anti-nucleocapsid (anti-N) antibodies, which are produced in response to infection but are not produced in response to COVID-19 vaccines currently authorized for emergency use or approved by the Food and Drug Administration. Drugs from the United States.
During September 2021 and February 2022, a convenience sample of blood samples submitted for clinical testing was analyzed for anti-N antibodies every 4 weeks; 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 SARS-CoV-2 antibody testing was ordered by the physician were excluded to reduce selection bias. During September 2021 and January 2022, the median sample size per 4-week period was 73,869 (range = 64,969–81,468); the sample size for February 2022 was 45,810.
Seroprevalence estimates were assessed for 4-week periods overall and by age group (0–11, 12–17, 18–49, 50–64, and ≥65 years). To produce estimates, the researchers weighted the results at the jurisdiction level relative to the population using an age-sex classification. CIs were calculated by bootstrap resampling ; statistical differences were assessed using nonoverlapping CIs. All samples were tested with the Roche Elecsys Anti-SARS-CoV-2 pan-immunoglobulin immunoassay. All statistical analyzes were performed using R statistical software (version 4.0.3; The R Foundation). This activity was reviewed by CDC, approved by the respective institutional review boards, and conducted in accordance with applicable federal law and CDC policy.
During September-December 2021, overall seroprevalence increased by 0.9 to 1.9 percentage points per 4-week period. During December 2021 to February 2022, overall US seroprevalence increased from 33.5% (95% CI = 33.1–34.0) to 57.7% (95% CI = 57.0–34.0). 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 of 0 to 11 years and from 45.6% (95% CI = 44.4–46.9) to 74.2% (95% CI = 72.8–75.5) among those 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 to 49 years, from 28, 8% (95% CI = 27.9–29.8) to 49.8% (95% CI = 48.5–51.3) among those aged 50–64, and 19.1% (95% CI = 18.4-19.8) to 33.2% (95% CI = 32.2-34.3) among those aged ≥65 years.
Figure: Seroprevalence of infection-induced SARS-CoV-2 antibodies,* by age group: United States, September 2021 to February 2022 
The findings of this report are subject to at least four limitations . First, convenience sampling could limit generalizability. Second, the lack of data on race and ethnicity prevented the weighting of these variables. Third, all samples were obtained for clinical testing and could overrepresent people with greater access to health care or who seek care more frequently. Finally, these findings could underestimate the cumulative number of SARS-CoV-2 infections because infections after vaccination could result in lower anti-N titers, and anti-N seroprevalence cannot explain reinfections.
As of February 2022, approximately 75% of children and adolescents had serological evidence of prior SARS-CoV-2 infection, and approximately one-third became seropositive since December 2021.
The largest increases in seroprevalence between September 2021 and February 2022 occurred in the age groups with the lowest vaccination coverage ; the proportion of the US population fully vaccinated by April 2022 increased with age (5 to 11, 28%; 12 to 17, 59%; 18 to 49, 69%; 50 to 64, 80%; and ≥65 years, 90%).
The lower seroprevalence among adults ≥65 years of age, who are at higher risk of severe illness from COVID-19, could also be related to the greater use of additional precautions with increasing age.
These findings illustrate a high rate of infection with the Omicron variant, especially among children. Seropositivity for anti-N antibodies should not be interpreted as protection against future infections . Vaccination remains the safest strategy to prevent complications of SARS-CoV-2 infection, including hospitalization of children and adults.
Vaccination against COVID-19 after infection provides additional protection against severe illness and hospitalization . Staying up to date with vaccination is recommended for all eligible individuals, including those with prior SARS-CoV-2 infection.
