SARS-CoV-2 Seroprevalence Study Shows Most Americans, Especially Children, Have Prior Evidence of Infection

Five months after the highly contagious Omicron variant of SARS-CoV-2, the virus that causes COVID-19, became predominant in the United States, 75% of children aged 17 and under show evidence of prior infection, according to national seroprevalence data released by the CDC.

The data, broken out by age group, found that more than 60% of adults up to age 49 have evidence of prior infection with SARS-CoV-2, the CDC said.

CDC Director Rochelle P. Walensky, MD, MPH, called the findings, published in the April 26 Morbidity and Mortality Weekly Report, an “important epidemiological study,” but she and other officials stressed that none of the agency’s recommendations—such as to stay up to date with vaccinations and boosters—have changed.

On a media call, Walensky and Kristie E.N. Clarke, CDR, MD, MSCR, FAAP, co-lead for the COVID-19 Epidemiology and Surveillance Taskforce Seroprevalence Team, strove to clarify what the report means and doesn’t mean.

The study uses data from CDC’s 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 to February 2022.

It estimates the proportion of the population that has antibodies from prior infection—and not from previous vaccinations, Walensky and Clarke stressed.

In addition, the laboratory findings are not able to discern the strength of the antibodies or how old they are, but in general, may go back as far as 1 to 2 years—essentially the span of the pandemic in the United States.

Sera are tested for anti-nucleocapsid (anti-N) antibodies, which are produced in response to infection.

During September 2021 to February 2022, a convenience sample of blood specimens submitted for clinical testing was analyzed every 4 weeks for anti-N antibodies.

During September 2021 to 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.

Age groups were 0 to 11, 12 to 17, 18 to 49, 50 to 64, and 65 years and older.

Overall US seroprevalence increased from 33.5% (95% CI, 33.1%-34.0%) to 57.7% (95% CI, 57.1%-58.3%).

By age group, seroprevalence increased:

• Among children aged 0 to 11 years, from 44.2% (95% CI, 42.8%-45.8%) to 75.2% (95% CI , 73.6%-76.8%)
• Among adolescents aged 12 to 17 years, from 45.6% (95% CI, 44.4%-46.9%) to 74.2% (95% CI, 72.8%-75.5%)
• Among adults aged 18 to 49 years, from 36.5% (95% CI, 35.7%-37.4%) to 63.7% (95% CI, 62.5%-64.8%)
• Among adults aged 50 to 64 years, 28.8% (95% CI, 27.9%-29.8%) to 49.8% (95% CI, 48.5%-51.3%)
• Among those 65 years and older, from 19.1% (95% CI, 18.4%-19.8%) to 33.2% (95% CI, 32.2%-34.3%)

One-third of the infections in children happened since December 2021, the report noted.