Median titer and frequency of neutralizing antibodies after second vaccination. (A) The mean titers of neutralizing antibodies were significantly lower in patients on dialysis and KTRs, compared with controls, after the second vaccination. (B) Frequencies of neutralizing antibodies in controls, patients on dialysis, and KTRs after the second vaccination. For comparison of three groups, data were analyzed by the nonparametric Kruskal–Wallis test with post hoc Dunn test. **P<0.01, ***P<0.001. NT, neutralization titer.