Circulating SARS-CoV-2 Spike IgG Antibody Levels and Avidity in Autoimmune, IBD and Transplant Cohorts: An Observational Study Following Multiple COVID-19 Vaccine Boosters

Abstract

Background: Individuals with autoimmune disease, inflammatory bowel disease (IBD), and transplants face a higher risk of severe COVID-19 and breakthrough hospitalizations. It is essential to understand the magnitude and durability of vaccine-induced humoral immune response in these populations. Methods: We evaluated SARS-CoV-2 spike IgG antibody levels and avidity in autoimmune, IBD, transplant, and healthy cohorts following multiple COVID-19 mRNA vaccine doses. Serum samples were collected approximately 1 month (8–52 days) and 6 months (158–202 days) post-vaccination. Antibody levels and avidity were measured using validated ELISA and chaotropic-based avidity assays. Results: Individuals with IBD and individuals with autoimmune disease, especially systemic autoimmune disease, exhibited lower antibody levels and avidity compared with healthy individuals at certain doses and time points. Transplant recipients demonstrated substantial impairments in both antibody levels and avidity, with avidity reduced across all doses and time points. Significantly lower avidity levels were observed in transplant recipients, suggesting challenges in developing or maintaining antibody quality. For example, geometric mean anti-spike IgG levels were substantially lower in transplant recipients than in healthy individuals at both 1 month (635 vs. 7685 BAU/mL; p < 0.0001) and 6 months (1146 vs. 3277 BAU/mL; p = 0.0057) post third dose. Similarly, transplant recipients had lower antibody avidity than healthy individuals after the third dose, with geometric mean AI80 values of 4.5 M vs. 5.5 M at 1 month (p < 0.0001) and 4.6 M vs. 5.4 M at 6 months (p < 0.0001). Age, sex, and vaccine manufacturer may further influence humoral immune responses in the transplant cohort. Conclusions: Vaccine-induced humoral immunity varies across autoimmune, IBD, and transplant cohorts, with the most persistent impairment observed in transplant recipients across vaccine dose groups and at both post-vaccination time points. These findings reveal immune response patterns that may guide future studies evaluating vaccination schedules, immune monitoring, and clinical outcomes in these populations.