Atlanta—While the headlines emphasized that the majority of people in the U.S. showed evidence of having been infected with COVID-19, a recent report from the CDC was more nuanced than that, especially when looking at specific age groups.

Essentially, past infection rates were about 75% for children but were only about a third for those aged 65 years and older—the group that remains most likely to be hospitalized or die from the infection.

The report in the CDC's Morbidity & Mortality Weekly Report noted that COVID-19 case rates peaked at their highest recorded levels after the B.1.1.529 (Omicron) variant of SARS-CoV-2 became predominant in the U.S. in December 2021.

CDC researchers used data from the agency's national commercial laboratory seroprevalence study to examine U.S. trends in infection-induced SARS-CoV-2 seroprevalence between September 2021 and February 2022 by age group. During that time, analysis was performed on a convenience sample of blood specimens submitted for clinical testing every 4 weeks. The focus was on anti-N antibodies.

The study reported that during September 2021 to December 2021, overall seroprevalence increased by 0.9 to 1.9 percentage points per 4-week period. During December 2021 to February 2022, overall U.S. seroprevalence increased from 33.5% (95% CI = 33.1-34.0) to 57.7% (95% CI = 57.1-58.3).

"Over 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 from 45.6% (95% CI = 44.4-46.9) to 74.2% (95% CI = 72.8-75.5) among persons aged 12-17 years," the CDC advised. "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 years, 28.8% (95% CI = 27.9-29.8) to 49.8% (95% CI = 48.5-51.3) among those aged 50-64 years, and from 19.1% (95% CI = 18.4-19.8) to 33.2% (95% CI = 32.2-34.3) among those aged >65 years."

A key factor was vaccination rates, noting that vaccination of the youngest groups of children still has not been authorized by the FDA.

About 75% of children and adolescents had serologic evidence of previous infection with SARS-CoV-2 as of February 2022, according to the report. A third of them had become newly seropositive since December 2021.

"The greatest increases in seroprevalence during September 2021–February 2022, occurred in the age groups with the lowest vaccination coverage; the proportion of the U.S. population fully vaccinated by April 2022 increased with age (5-11, 28%; 12-17, 59%; 18-49, 69%; 50-64, 80%; and >65 years, 90%)," the CDC authors explained. "Lower seroprevalence among adults aged >65 years, who are at greater risk for severe illness from COVID-19, might also be related to the increased use of additional precautions with increasing age."

One key finding was the especially high infection rate for the Omicron variant, especially among children. Researchers cautioned, however, that seropositivity for anti-N antibodies should not be interpreted as complete protection from future infection.

"Vaccination remains the safest strategy for preventing complications from SARS-CoV-2 infection, including hospitalization among children and adults," the authors noted. "COVID-19 vaccination following infection provides additional protection against severe disease and hospitalization. Staying up to date with vaccination is recommended for all eligible persons, including those with previous SARS-CoV-2 infection."

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