Low Peripheral B-Cell Counts in Patients With Systemic Rheumatic Diseases Due to Treatment With Belimumab and/or Rituximab Are Associated With Low Antibody Responses to Primary COVID-19 Vaccination.

Background

Immunosuppressive agents inhibit COVID-19 vaccine antibody (Ab) responses in patients with systemic rheumatic diseases. Rituximab may fully block Ab responses when B cells become undetected. The effect of detected but low number of B cells due to treatment with a B-cell agent (belimumab and/or rituximab) has not been established. Purpose: We sought to examine whether there is an association between a low number of B cells due to treatment with belimumab and/or rituximab and impaired primary COVID-19 vaccination spike Ab responses in patients with systemic rheumatic diseases.

Methods

We retrospectively examined Ab responses to COVID-19 vaccinations, especially in relation to B-cell counts after treatment with belimumab and/or rituximab, in 58 patients with systemic rheumatic diseases: 22 on and 36 not on B-cell agents. We used Kruskal-Wallis and Mann-Whitney U tests for comparison of Ab values between the groups and Fisher exact test for relative risk calculations.

Results

Median (interquartile range) postvaccination Ab responses were lower in patients on versus those not on B-cell agents: 3.91 (0.77-20.00) versus 20.00 (14.32-20.00), respectively. Among patients on belimumab and/or rituximab, Ab responses of less than 25% of the assay's upper limit were exclusively observed in those with B-cell counts lower than 40/µL. Patients with B-cell counts lower than 40/µL exhibit a relative risk of 6.092 (95% CI: 2.75-14.24) for Ab responses of less than 25% of the upper limit compared with patients not on B-cell agents. This relative risk remained significant, even after excluding patients with undetected B cells.

Conclusion

This retrospective study found an association between low B-cell counts (less than 40/µL) and decreased Ab responses to primary COVID-19 vaccination in patients with systemic rheumatic diseases treated with belimumab and/or rituximab. Despite the small number of patients studied, these findings add to the accumulating evidence on the importance of B-cell count in predicting spike Ab responses to COVID-19 vaccination.