The aim of this investigation was to determine the effect of SARS-Cov-2 vaccination in hemodialysis patients, search for risk factors for non- or low-response, and to measure the effect of a third booster vaccination in non- or low-responders. Methods SARS-CoV-2 IgG antibodies and the virus-neutralizing capacity were measured 4–5 weeks after a full standard vaccination in 95 chronic hemodialysis patients and 60 controls. IgG titers > 30 AU/mL served to classify participants as responders. Multivariable binary logistic regression analysis was used to search for risk factors of reduced vaccination success. Patients with vaccination failure were offered a third booster dosage. Results 82.1% of the patient cohort as compared to 98.3% of the healthy control group were able to mount SARS-CoV-2 titers above 30 AU/mL after two standard vaccine doses. Mean IgG antibody titers were lower in hemodialysis patients than controls (78 ± 35 vs. 90 ± 20 AU/mL, p = 0.002). Multivariable binary logistic regression analysis showed age and immunosuppressive medication as major risk factors for vaccination failure with a decreased probability of successful vaccination of −6.1% (95% CI −1.2 to −10.9) per increase in age of one year and −87.4% (95% CI −98.0 to −21.5) in patients on immunosuppressive therapy (crude odds ratio for vaccination failure for immunosuppressive therapy 6.4). Ten out of 17 patients with non-response to vaccination were offered a third dose. Booster vaccination after the second dose induced an increase in effective antibody titers of >30 AU/mL in seven out of ten patients 4–5 weeks later (70%). Conclusion Standard SARS-CoV-2 vaccination schemes are highly effective in mounting protective neutralizing IgG antibodies in chronic hemodialysis patients. Nevertheless, response to vaccination is diminished as compared to a healthy control group. Major risk factors for vaccination failure are older age and immunosuppressive therapy. In non- or low-responders to standard vaccination a third booster vaccination was able to induce effective antibody titers in about 70% of patients, indicating that a third booster vaccination might be preferable to decreasing immunosuppressive therapy.

The aim of this study is to determine the effect of repeated vaccinations on neutralizing SARS-CoV-2 IgG antibody titers, evaluate risk factors for immunological non-response, and to report breakthrough infections in chronic hemodialysis patients. Methods: A prospective, multi-center cohort study in 163 chronic hemodialysis patients was conducted. Antibody titers were measured three months after second, third, and fourth (10 pts) booster vaccinations. SARS-CoV-2 neutralizing antibody titers in BAU/mL and % inhibition were divided into three categories (<216, 216–433, >433 and <33, 33–66, and >66%). Somers’s test, paired t-test, and univariable and multivariable logistic regression analysis were applied to evaluate differences in antibody levels and search for risk factors for vaccination failure defined as neutralizing titers <50% and/or need for repeated booster vaccinations. Furthermore, we report on a case series to describe characteristics of patients after four vaccinations (n = 10) and breakthrough infections (n = 20). Results: Third dose boosters resulted in higher proportions of patients with neutralizing antibody levels >66% as compared to after the second dose (64.7% after second dose vs. 88.9% after third dose, p = 0.003), as well as in a respective increase in neutralizing titer levels in % from 68 ± 33% to 89 ± 24 (p < 0.001). The proportion of patients with IgG-titers below 216 BAU/mL decreased from 38.6 to 10.5% (p ≤ 0.001). Age (p = 0.004, OR 1.066, 95% CI 1.020–1.114) and presence of immunosuppressive medications (p = 0.002, OR 8.267, 95% CI 2.206–30.975) were identified as major risk factors for vaccination failure. Repeated booster vaccinations ≥4 times were effective in 8 out of 10 former low-responders (80%) without any side effects or safety concerns. Breakthrough infections showed a clinically mild course but were associated with prolonged viral shedding on PCR-testing ranging 7–29 (mean 13) days. Conclusions: Third and fourth mRNA-based booster vaccinations resulted in higher and longer lasting SARS-CoV-2 antibody levels as compared to after two dosages. The presence of immunosuppressive medication and repeat vaccinations are major potentially modifiable measures to increase antibody levels in non-or low-responders. Breakthrough infections with SARS-CoV-2 Omicron were associated with prolonged viral shedding but clinically mild disease courses.