Introduction:
Patients undergoing haemodialysis (HD) have been recommended to receive booster doses of mRNA vaccines against SARS-CoV-2 as part of the group of persons at risk for severe sequelae of coronavirus disease (Covid-19).

Question:
The aim of this study was to longitudinally analyse the humoral immune response to spike protein and the impact breakthrough infections (BTI) in HD.

Materials & Methods:
82 HD and 38 healthy controls (HC) had received a total of 3 to 5 mRNA vaccinations. Blood samples were taken before and at 2 and 4 weeks and at 3, 6 and 9 months after vaccination. The concentration anti-spike-IgG (binding antibody units, BAU/ml) and the relative avidity index (%) were assessed by adaption of a commercially available ELISA assay using thiocyanate as chaotropic reagent.

Results:
HD who had received a 5th vaccination 6 months after the 4th vaccination showed a higher anti-spike-IgG (mean 3335,35 BAU/ml) after 6 months compared to HD who had received only 4 vaccinations and were evaluated at time point 12 months after the 4th vaccination (2344,83 BAU/ml). The RAI remained similar between HD who were vaccinated five times and those who had 4 vaccinations (76,64% versus 79,33%) with no significant difference compared to HC (78,08%). HD with BTI in the last 6 months showed 1,57-fold higher anti-spike-IgG compared to those without BTI.

Conclusion:
The results revealed an immunological benefit of a fifth vaccination for HD patients by higher anti-spike-IgG concentrations. Hybrid immunity by BTI was able to further induce anti-spike-IgG. The findings corroborate the recommendation for repeated COVID19 mRNA vaccinations in HD patients.