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The 2021 COVID-19 Omicron wave in the U.S. caused record-high case counts, but underreporting of cases (due to asymptomatic infections and use of at-home testing) limits our understanding of the true population-level effect. In this study, researchers at the U.S. CDC used a nationally representative sample of blood specimens tested at commercial labs (for reasons unrelated to COVID-19) to estimate the proportion of all individuals with antibodies to the SARS-CoV-2 nucleocapsid, indicating natural infection. Convenience samples of 45,000 to 75,000 specimens were tested every 4 weeks from September 2021 to February 2022 to generate age-specific seroprevalence estimates. The major findings were:
Across all age groups, seroprevalence increased from 34% in December 2021 to 58% in February 2022.
The largest increases occurred in children (age range, 0–11; from 44% to 75%) and adolescents (age range, 12–17; from 46% to 74%).
By February 2022, three quarters of children and adolescents had been infected with SARS-CoV-2, with about one third of these infections occurring during the Omicron wave.
Clarke KEN et al. Seroprevalence of infection-induced SARS-CoV-2 antibodies — United States, September 2021–February 2022. MMWR Morb Mortal Wkly Rep 2022 Apr 29; 71:606. (https://www.cdc.gov/mmwr/volumes/71/wr/mm7117e3.htm)
Comment
These results highlight Omicron's profound infectiousness. If anything, this study likely underestimates population prevalence, because of the roughly 2-week time lag between infection and antibody detection (people with recent infection might test negative). Further, the antibody test used is highly specific, making false positives very unlikely. Because the extent to which antibody positivity protects against future infection is unknown, vaccination is recommended for all eligible people, regardless of prior infection.