(A) Humoral responder rates were determined based on serum samples collected on day 8 ± 1 after boost being analyzed for spike S1 domain–specific IgG (left, Fisher’s exact test) and IgA (middle, Fisher’s exact test) by ELISA. Surrogate virus neutralization capacity was assessed by a blocking ELISA (right, Fisher’s exact test) with HC (n = 39), KTx (n = 39), and HD (n = 26). (B) Sera of KTx patients available from day 23 ± 5 after boost immunization were retested for reactivity as in A with n = 24. (C) Serological reactivity was quantified only in responding individuals on day 8 ± 1 after boost. IgG, Mann-Whitney U test: HC, n = 39; KTx, n = 1; HD, n = 22. IgA, Kruskal-Wallis test: HC, n = 38; KTx, n = 0; HD, n = 21. Neutralization, Mann-Whitney U test: HC, n = 39; KTx, n = 0; HD, n = 20). NA, not applicable due to nonresponsiveness. Graphs show mean ± SD.