Serologic testing of US adults finds that nearly 42% have SARS-CoV-2 antibodies indicating previous infection, but about 44% of them said they never had COVID-19, according to a study published today in Morbidity and Mortality Weekly Report.
Centers for Disease Control and Prevention (CDC) and Westat Corp researchers analyzed serologic testing data from a sample of 1,574 participants in the National Health and Nutrition Examination Survey (NHANES) from August 2021 to May 2022, a period spanning the Delta and Omicron variant waves.
Asymptomatic infection likely
Preliminary, unweighted NHANES results showed that 91.5% of adults had SARS-CoV-2 anti-spike antibodies (indicating previous infection or vaccination), and 41.6% had anti-nucleocapsid antibodies (indicating previous infection only).
But 43.7% of those with evidence of infection said they never had COVID-19, suggesting asymptomatic (symptom-free) infection. Of adults with antibodies to the virus, 25.5% said they were unvaccinated against COVID-19, meaning that their antibodies originated from an infection.
Rates of seropositivity declined with advancing age (59.7% of adults 18 to 29 years old vs 30.2% aged 70 and older). Seropositivity also varied by race, with 59.2% of Hispanic, 45.9% of Black, and 30.6% of White adults infected and possibly vaccinated. Rates also declined by educational attainment, with 49.0% of those with less than a high school education versus 37.5% of those with at least some college infected and possibly vaccinated.
In contrast, the proportion of adults with anti-spike but not anti-nucleocapsid antibodies (vaccinated, not infected) was 49.9% overall, rose with age (28.1% among 18- to 29-year-olds vs 64.7% among those 70 and older), was lower among Hispanic (35.3%) and Black participants (46.7%) and higher in White adults (58.9%), lower in those without a high school diploma (42.5%), and higher in those with at least some college (55.4%).
Rates of positivity for antibodies indicating previous infection were highest in Black adults (57.1%) and in those with less than a high school education (57.8%). Proportions of unvaccinated respondents fell with age (31.6% among aged 18 to 29 years vs 18.8% among those 70 and older). A higher percentage of Black participants (31.3%) and a lower percentage of Hispanic adults (21.4%) with antibodies indicating infection said they were unvaccinated.
Health equity concerns
"Younger adults and Black adults with unidentified infections might have been more likely to lack access to testing and to have unknowingly exposed others, resulting in disparities in community transmission," the study authors wrote. "In this way, undiagnosed infections could have amplified disparities in infection rates and outcomes."
The researchers recommended that Americans stay current with COVID-19 vaccinations. "Consistent with findings from other seroprevalence studies, preliminary NHANES 2021-2022 results raise health equity concerns given the disparities observed in SARS-CoV-2 infection and COVID-19 vaccination," they concluded.
"These results can guide ongoing efforts to achieve vaccine equity in COVID-19 primary vaccination series and booster dose coverage."