By Ethan Covey
A CDC report determined that 96.4% of U.S. residents ages 16 years and older have SARS-CoV-2 antibodies, from previous infection or vaccination.
Additionally, almost half of the population has hybrid immunity, derived from both previous infection and vaccination, which has been shown to provide increased protection over infection or vaccination alone. However, hybrid immunity remains lowest among adults 65 years and older—the population most at risk for severe disease (MMWR Morb Mortal Wkly Rep 2023;72:601-605).
The study focused on data from the CDC, which has been tracking estimated SARS-CoV-2 seroprevalence through blood donations since July 2020.
In the second quarter of 2021, an estimated total of 68.4% of people ages 16 and older had infection- or vaccination-induced SARS-CoV-2 antibodies. This group included 47.5% with antibodies from vaccination alone, 12% from infection alone, and 8.9% from both infection and vaccination. By the third quarter of 2022, these numbers had risen significantly: 96.4% had SARS-CoV-2 antibodies from previous infection or vaccination, including 22.6% from infection alone and 26.1% from vaccination alone; 47.7% had hybrid immunity.
Hybrid immunity was lowest among people 65 years and older (36.9%), and highest among those 16 to 29 years (59.6%).
“The proportion of people with hybrid immunity has increased over time,” said Jefferson M. Jones, MD, a medical officer with the CDC. “Hybrid immunity was lowest among older adults because of lower rates of infection, likely due to higher rates of vaccination and preventive behaviors.”
However, Dr. Jones noted that older adults have consistently been at increased risk for severe illness from COVID-19, and “lower rates of infection-induced and hybrid immunity could further increase the risk for severe disease in this group, while also highlighting the importance for adults ages 65 years and older to stay up-to-date with COVID-19 vaccination and have easy access to antiviral medications.”
The study also found that unvaccinated people had higher rates of infection than vaccinated people, suggesting that vaccination provides some protection against infection.
From April-June 2021 through January-March 2022, the incidence of first SARS-CoV-2 infections among unvaccinated people was 67%, compared with 26.3% among vaccinated people. From January-March 2022 through April-June 2022, the incidence among unvaccinated people was 21.7% and was 13.3% among vaccinated people. For April-June 2022 and July-September 2022, the incidence among unvaccinated people was 28.3%, compared with 22.9% among vaccinated people.
Dr. Jones said the differences in incidence could also be due to systematic differences between vaccinated and unvaccinated people in terms of practicing prevention behaviors like masking and physical distancing.? Future studies may help clarify these differences.
“Although COVID-19 booster vaccine doses and reinfections can strengthen immunity, this analysis did not account for these effects because blood donor vaccination history did not include the number of doses received, and data on reinfections were not captured,” Dr. Jones said. “Immunity wanes over time, but time since vaccination or infection was not included in the analysis. Models and analyses that incorporate these factors could better estimate population-level protection against infection and severe disease.”
The sources reported no relevant financial disclosures.
