Statistical analyses
We used IBM SPSS Statistics version 23.0 (SPSS Inc., Chicago, USA) and Prism 8.0 (GraphPad Software Inc., San Diego, USA) to analyze the data. Normally distributed data are presented as mean ± standard deviation (SD). Data with a skewed distribution are presented as median (interquartile range [IQR]). Categorical data are expressed as number (percentage).To assess changes between baseline and post-vaccination measurements, we used the paired T-test for normally distributed data or the Wilcoxon-signed rank test for data with a skewed distribution. To assess differences between the two study arms at four weeks after the successive vaccinations, the independent samples T-test and Mann-Whitney Test were used. To adjust for differences in baseline lymphocyte count or eGFR, analysis of covariance was performed. Correlations were analyzed using the Spearman’s rank test.The primary analyses were performed according to the intention-to-treat principle. We additionally performed ten sensitivity analyses. First, a per-protocol analysis was performed, excluding KTRs who were still iron-deficient after treatment with FCM, as well as KTRs who were not iron-deficient despite placebo treatment. In a second sensitivity analysis, we excluded four patients who were seropositive for total SARS-CoV-2-specific anti-RBD antibodies (n=3) or anti-SARS-CoV-2 IgG antibodies (n=1) at baseline. As a third sensitivity analysis, we excluded KTRs who were IgG-deficient at baseline. Fourth, we repeated the analyses in a subpopulation restricted to patients who had received the mRNA-1273 vaccine. In a fifth sensitivity analysis, only patients with severe ID (ferritin <30 μg/L) were included. Sixth, we analyzed patients on dual or triple immunosuppressive therapy separately. Seventh, patients who had received treatment with alemtuzumab, methylprednisolone or anti-thymocyte globulin <2 years prior to vaccination were excluded. In an eighth sensitivity analysis, the subgroup of patients who used mycophenolic acid was studied. Moreover, the subgroup of patients using mycophenolic acid with a dosage of 500mg twice daily was analyzed separately. Finally, we analyzed men and women separately. In all analyses, a P value of ≤0.05 was considered significant.