At a glance
Antibodies are proteins that your immune system makes to help fight infection and protect you from getting sick in the future. Antibodies to SARS-CoV-2, the virus that causes COVID-19, can be detected in the blood of people who have recovered from COVID-19 or people who have been vaccinated against COVID-19.
Antibodies and COVID-19
Some people with antibodies to SARS-CoV-2 may become infected after vaccination (vaccine breakthrough infection) or after recovering from a past infection (reinfected). When reinfections or breakthrough infections happen, having antibodies plays an important role in helping prevent severe illness, hospitalization, and death.
For many diseases, including COVID-19, antibodies are expected to decrease or "wane" over time. As antibodies wane, you may become more vulnerable to severe illness.
How scientists study antibodies
The science of antibodies is called "serology." Antibody tests, also called "serology tests," identify antibodies in blood samples. While other parts of the immune system also contribute to protection, it is easiest to test for antibodies.
Seroprevalence surveys are studies that involve the use of serology tests (also known as antibody tests) to better understand how many people have SARS-CoV-2 antibodies from prior vaccination, infection, or both. These surveys also allow scientists to study antibodies at different points in times, in different locations, and in different populations in the United States.
Many studies have associated higher levels of SARS-COV-2 antibodies with higher protection from infection. Future studies will focus on:
- how many have antibodies people have
- whether antibody levels are going up or down
- what levels of antibodies are needed to protect from infection and severe disease.
What we can learn from seroprevalence
Seroprevalence studies inform our understanding of the epidemiology of COVID-19. Tracking population seroprevalence over time, in a variety of specific geographic sites, can inform models of virus transmission and policy decisions regarding the impact of physical distancing and other preventive measures.
Estimated COVID-19 seroprevalence in United States
Key Takeaways:
- Studies to measure seroprevalence of antibodies from prior SARS-CoV-2 infection and COVID-19 vaccination were conducted during 2020-2023.
- Both vaccine-induced and infection-induced seroprevalence increased over time.
- By the end of 2023, SARS-CoV-2 antibodies were detected in over 95% of adults and 90% of children.
- Young children are most likely to lack antibodies to protect against COVID-19. Parents of children ages 6 months-17 years should discuss the benefits of vaccination with a healthcare provider.
- The presence of antibodies does not necessarily protect from future infection or severe disease. Protection from prior infection or vaccination wanes with time. Getting an updated vaccine, when recommended, can provide additional protection against COVID-19.
Commercial Laboratory National Seroprevalence Study
Caption: Data Source: Commercial Laboratory National Seroprevalence Study
Footnote: Methods for the national commercial laboratory seroprevalence study conducted during October 2020-Feburary 2022 have been . Beginning in March 2022, the study changed to estimated seroprevalence by 8-week information collection periods and focus on pediatric patients only. Beginning in March 2022, antibody information for adults relied on blood donor seroprevalence studies.
Footnote: Methods for the national commercial laboratory seroprevalence study conducted during October 2020-Feburary 2022 have been . Beginning in March 2022, the study changed to estimated seroprevalence by 8-week information collection periods and focus on pediatric patients only; historical monthly data for the U.S. pediatric population from September 2021 through February 2022 are provided for context. Beginning in March 2022, antibody information for adults relied on blood donor seroprevalence studies. Antibody information for adults relied on blood donor seroprevalence studies.
Data Source: Commercial Laboratory National Seroprevalence Study
Footnote: Methods for the national blood donor seroprevalence study conducted during 2020-2021 as a repeat, cross-sectional survey have been . During 2022, this study was conducted as a longitudinal study of ~143,000 blood donors, and during 2023, the study continued among ~35,000 blood donors.
For each quarter, one randomly selected blood donation is selected per donor and tested for COVID-19 antibodies. For each quarter in 2022-2023, one randomly selected blood donation was selected per donor and tested for anti-spike antibodies using the Ortho VITROS® SARS-CoV-2 quantitative IgG assay and anti-nucleocapsid antibodies using the Ortho VITROS SARS-CoV-2 total antibody assay.
Until December 2021, blood specimens were tested for anti-spike antibodies using the Ortho VITROS® SARS-CoV-2 total antibody assay, which was able to detect low levels of antibodies years after infection or vaccination. The assay used in 2022 better quantifies the level of antibody in the blood but does not detect low levels of antibodies. For these reasons, the change in anti-spike antibody assay might result in lower 2022 anti-spike estimates compared with 2021 estimates.
Footnote: Methods for the national blood donor seroprevalence study conducted during 2020-2021 as a repeat, cross-sectional survey have been . During 2022, this study was conducted as a longitudinal study of ~143,000 blood donors, and during 2023, the study continued among ~35,000 blood donors.
For each quarter, one randomly selected blood donation is selected per donor and tested for COVID-19 antibodies. For each quarter in 2022-2023, one randomly selected blood donation was selected per donor and tested for anti-spike antibodies using the Ortho VITROS® SARS-CoV-2 quantitative IgG assay and anti-nucleocapsid antibodies using the Ortho VITROS SARS-CoV-2 total antibody assay.
Until December 2021, blood specimens were tested for anti-spike antibodies using the Ortho VITROS® SARS-CoV-2 total antibody assay, which was able to detect low levels of antibodies years after infection or vaccination. The assay used in 2022 better quantifies the level of antibody in the blood but does not detect low levels of antibodies. For these reasons, the change in anti-spike antibody assay might result in lower 2022 anti-spike estimates compared with 2021 estimates.
- Wiegand, R. E., Deng, Y., Deng, X., Lee, A., Meyer, W. A., 3rd, Letovsky, S., Charles, M. D., Gundlapalli, A. V., MacNeil, A., Hall, A. J., Thornburg, N. J., Jones, J., Iachan, R., & Clarke, K. E. N. (2023). , United States-October 25, 2020-February 26, 2022. Lancet regional health. Americas, 18, 100403.