LAUSR

To the Editor: COVID-19–related mortality in kidney transplant recipients (KTX) is significantly higher when compared with nontransplant patients. Vaccination against SARS-CoV-2 is the most efficient measure to protect this vulnerable population. Nevertheless, after the application of 2 doses of mRNA-based vaccine, more than 50% of kidney transplant recipients have seronegative results. A third dose of the mRNA-based vaccination induced seroconversion in 60% of the primary nonresponders, which however still leaves approximately 25% of all KTX without protection. Because heterologous mRNA and vector-based COVID-19 vaccine regimens were found to be superior compared with homologous regimens, we hypothesized that a fourth vaccination with a vectorbased vaccine can improve humoral SARS-CoV-2 immunity in KTX who received 3 doses of COVID-19 vaccination without having a humoral response. A cohort of 20 KTX under immunosuppression (Supplementary Table S1) with low or no SARS-CoV-2– specific IgG 4 weeks after mRNA-based vaccinations received a fourth dose of the vector-based vaccine Ad26.COV2.S, which was well tolerated. The vaccination induced seroconversion in only 2 of 13 seronegative KTX. In KTX with increasing antibody titers, the median titer developed from 3.7 [0.6–35.9] to 50.0 [11.3–342.9]. Of the 9 KTX with detectable IgG titers, a borderline neutralizing capacity against the WT SARSCoV-2 strain and the delta VOC could be observed in 3 and 4 KTX, respectively (Figure 1a and b). The functional SARS-CoV-2–reactive CD4 and CD8 T-cell response could be enhanced in approximately 30% to 60% of the KTX (Figure 1c–i). T-cell immunity against WT-S and Delta-VOC-S was comparable (Figure 1m and n). The CD4 T-cell response did correlate neither with