Statistical analysis was performed using the SPSS program v. seropositive patients were higher in the heterologous group (7771 versus 599 AU/mL; < 0.0001). These results persisted after adjusting for confounding variables. Lastly, a ATP (Adenosine-Triphosphate) similar proportion of patients became seropositive for neutralizing antibodies (98% versus 94%; = 0.098). Conclusions. In kidney transplant recipients fully vaccinated with CoronaVac, a third dose with an mRNA vaccine produced a higher seroconversion rate and antibody titers than a third homologous dose. However, both boosters achieved comparative seroprevalence for neutralizing antibodies. The high proportion of still seronegative patients indicates the need for alternate strategies of protection. INTRODUCTION Kidney transplant recipients accomplish seroprevalence rates between 3% and ATP (Adenosine-Triphosphate) 59% 4 wk after 2 doses of mRNA, viral vector, or inactivated whole-virion vaccines,1-4 which is lower than the general populace.5,6 Developing a humoral immune response is associated with protection against breakthrough infections because more severe disease and deaths concentrate among seronegative patients.3,4,7 Thus, it seemed reasonable to apply additional doses of vaccine to achieve immunization in transplanted patients, who are at higher risk for coronavirus disease 2019 (COVID-19)Crelated mortality.8 The first attempts were to administer homologous boosters to seronegative patients after the full 2 doses of vaccination. However, in a French cohort, only 44% of the 59 solid organ transplant recipients who had been seronegative developed antibodies after a third homologous dose of the BNT162b2 mRNA vaccine.9 Similarly, we have observed a low seroconversion rate of 20.3% after a third homologous dose of the CoronaVac vaccine in ATP (Adenosine-Triphosphate) those seronegative kidney transplant recipients.10 In the general populace, initiated from safety considerations associated with the Oxford-AstraZeneca ChAdOx1-S (AZD1222) vaccine, mixing with mRNA vaccines ultimately has produced evidence in favor of a similar or even superior humoral and cellular response and clinical effectiveness compared to those with the homologous prime boosters.11-15 In the transplant populace, a small German study including 40 kidney transplant recipients and 70 immunocompetent controls found that heterologous boosting with mRNA after vector vaccine priming was the regimen that produced the most robust humoral and cellular response.16 The little experience on heterologous boosters in kidney transplant recipients is restricted to endogenous antigen platform vaccines, either viral vector or mRNA. Theoretically, exposure to multiple viral antigens from an inactivated computer virus vaccine could trigger a stronger induction of neutralizing antibodies. Additionally, you will find few studies systematically comparing the effect of a heterologous versus a homologous third dose. In this analysis, we compared the seroconversion rates after a heterologous third dose of the BNT262b2 mRNA vaccine versus a homologous third dose of the CoronaVac inactivated whole-virion vaccine after the former 2-dose routine of CoronaVac. Additionally, we evaluated the increased rate in antibody titers and neutralizing activity after heterologous BNT162b2 versus homologous CoronaVac third doses. We also explained the occurrence and severity of breakthrough infections up to 3 mo after the third dose. Finally, we investigated independent factors for seroconversion after the third dose, high antibody values, and the occurrence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) contamination after vaccination. MATERIALS AND METHODS Study Design This is a prospective single-center cohort study derived from the previously published initial trial in that kidney transplant recipients received 2 doses of the inactivated CoronaVac vaccine.17 The conception of the present study was driven by the urge of giving to the largest number Rabbit Polyclonal to BATF of patients as soon as possible an additional vaccine dose against coronavirus, following the National Guidelines. Therefore, no prior hypothesis was conceived, and no sample size was decided on advance. This study protocol was approved by the local ethics committee, and the patients signed an updated informed consent form. Inclusion criteria consisted of kidney transplant recipients aged 30C69 y, with >30 d of transplant, who experienced received the standard schedule of 2 doses of the CoronaVac vaccine at the transplant center between March 20 and April 25, 2021, and had been included in the initial study. Exclusion criteria were using a confirmed COVID-19 after vaccination, allograft loss, death, or ATP (Adenosine-Triphosphate) loss of follow-up. Additionally, to avoid large displacements and increased exposure to SARS-CoV-2 infection, and considering that the additional vaccine doses were already available in the public health system, only patients living in the city of Sao Paulo were approached. Those who lived abroad.