Background Antibody response against SARS-CoV-2 after mRNA or adenoviral vector based vaccines is weak in kidney transplant (KT) patients. However, few studies have focused on humoral response after inactivated virus-based vaccines in KT. Here, we compare antibody response following vaccination with inactivated virus (Coronavacࣨ) and BNT162b2 mRNA. Methods A national multicenter cross-sectional study was conducted. The study group was composed of patients from all KT centers in Uruguay, vaccinated between May 1st-May 31st (Coronavacࣨ n = 245 and BNT162b2 n = 39). Control group was constituted by 82 healthy individuals. Participants had no prior confirmed COVID-19 test. Blood samples were collected between 30 and 40 days after second dose. Serum specific IgG antibodies against Receptor Binding Domain (RBD) of SARS-CoV-2 Spike protein were determined using COVID-19 IgG QUANT ELISA Kit. Results Only 29% of KT recipients showed seroconversion [36.5% BNT162b2, 27.8% inactivated virus, p = 0.248] in comparison to 100% in healthy control with either vaccine. Antibody levels against RBD were higher with BNT162b mRNA than with inactivated virus [173 (73–554) and 29 (11–70) BAU/mL, p < 0.034] in KT and 10 times lower than healthy control [inactivated virus: 308 (209–335) and BNT162b2: 2638 (2608–3808) BAU/mL, p < 0.034]. In multivariate analysis, variables associated with negative humoral response were age, triple immunosuppression, eGFR and time post-KT. Conclusion Seroconversion was low in KT patients after vaccination with both platforms. Antibody levels against SARS-CoV-2 were lower with inactivated virus than BNT162b mRNA. These findings support the need of strategies to improve immunogenicity in KT recipients after two doses of either vaccine.

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