*Purpose: This study aims to compare the immunogenicity of a third dose of the heterologous BNT262b2 mRNA vaccine versus the homologous inactivated whole-virion CoronaVac vaccine in adult kidney transplant recipients (KTR).

*Methods: This prospective, single-center, phase 4 interventional study included KTR aged 30-69 years, with more than 30days of transplantation, and no previous confirmed COVID-19. The patients received the 3rd heterologous (BNT162b2 mRNA) or homologous dose, at least four weeks after the standard two-dose schedule of CoronaVac vaccine, at the transplant center. Antibody response immediately before and after the 3^rd dose was assessed by the AdviseDx SARS-CoV-2 IgG II assay. For those positive assays, neutralizing anti-SARS-CoV-2 antibodies were assessed through the cPass™ SARS-CoV-2 Neutralization Antibody Detection Kit.

*Results: There were 307 patients in the heterologous group and 777 patients in the homologous group. KTR in the heterologous group were older (median age 54 vs. 50 years,p<0.0001), with a lower prevalence of diabetes (7% vs. 11%,p=0.032), lower percentage of deceased donors (60% vs. 68%,p=0.006) and longer time since transplant (median 11 vs. 6 years,p< 0.0001).Immediately before the 3^rd dose, seroprevalence for IgG antibodies (36% vs. 34%,p=0.597) and the median antibody titers among those seroprevalent (246 AU/mL vs. 268 AU/mL,p=0.279) were similar. At a median of 25 days after the heterologous and 35 days after the homologous 3^rddose vaccine, seroconversion rate was higher in the heterologous group (49% vs. 32%,p<0.0001), resulting in a higher seroprevalence rate (67% vs. 55%,p=0.0003). Overall, 42% remained seronegative after the third dose. The median antibody titers after booster among those seroprevalent patients was higher in those in whom the 3^rd heterologous vaccine was administered (7,771 AU/mL vs 599 AU/mL,p<0.0001). The analysis of the neutralizing activity is ongoing.

*Conclusions: This prospective interventional study suggests that a 3^rd heterologous dose is associated with a higher seroconversion rate and median antibody titers compared to a homologous dose in kidney transplant recipients fully vaccinated with inactivated whole-virion CoronaVac vaccine. In addition, 42% of subjects did not produce humoral immune response after the third dose, urging the development of alternative strategies.

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