B-119 Seroprevalence of SARS-CoV-2 IgG Spike and Nucleocapsid Antibodies in the Northeastern United States: 2023-2024

Abstract Background The first vaccines against SARS-CoV-2 were introduced in early 2021, representing a significant milestone in public health. Monitoring the levels of SARS-CoV-2 antibodies in blood has been critical for understanding immune responses due to either infection or vaccination. Initial studies indicated that IgG anti-spike and IgG anti-nucleocapsid antibodies persisted strongly in patient sera for up to 1 year following infection or immunization. However, the percentage of people receiving vaccinations against SARS-CoV-2 has been low despite the ongoing availability of yearly booster shots. The Centers for Disease Control and Prevention estimated that at the end of 2023, only 22% of adults and 7.5% of children had received the updated SARS-CoV-2 booster vaccine. At the end of 2024, vaccine uptake was estimated to be 23% among adults and 11% among children. In this study, we conducted a retrospective analysis of serological testing results to track the percentage of specimens with detectable levels of SARS-CoV-2 IgG anti-spike and IgG anti-nucleocapsid antibodies. Methods This retrospective analysis included all serum specimens submitted for SARS-CoV-2 serological testing to a Quest Diagnostics laboratory in the northeastern United States from January 2023 through December 2024. Immunoassays were used to detect the presence of IgG anti-spike and anti-nucleocapsid antibodies. We monitored monthly positivity rates for 2023 and 2024. Results IgG anti-spike antibody positivity rates remained consistently high throughout the entire 24-month period, never falling below 95%. Furthermore, more than 60% of these specimens exhibited titers exceeding the upper measurement limit (>1:2500). In contrast, IgG anti-nucleocapsid antibody positivity varied more widely, showing rates between approximately 20% and 60% during the study period. Conclusion Despite the declining vaccination rates among the general population, positivity for IgG anti-spike antibodies remains high during the study period among patients undergoing serological testing. IgG anti-nucleocapsid positivity was lower and varied more, consistent with the fact that most vaccines target the spike protein. However, positive results for IgG anti-nucleocapsid antibodies can indicate either a recent or past infection with SARS-CoV-2.