Background Kidney transplant recipients and patients receiving hemodialysis are immunocompromised populations that are prioritized for COVID-19 vaccination but were excluded from clinical trials of SARS-CoV-2 mRNA vaccines. Antibody titers and rates of seroconversion following vaccination are lower among patients with chronic kidney disease and those taking immunosuppressants compared with controls. Data are lacking regarding their humoral response to vaccination to prevent COVID-19. Methods This investigation of early serological response after COVID-19 vaccination with the Pfizer/BioNTech (BNT162b2) mRNA vaccine included 78 patients undergoing hemodialysis, 74 kidney transplant recipients, and 7 healthy controls. We recorded data from the medical file for various clinical parameters, including response to hepatitis B vaccination, and measured antibody titers against SARS-CoV-2 at 0, 14, 28, 36 and 58 days after the first injection. Results In controls, we detected antibodies at a positive level (>13 arbitrary units per milliliter [AU/ml]) at day 14 postinjection, which increased progressively to peak at day 36 (1082 AU/ml; interquartile range [IQR], 735.0-1662.0]). Patients undergoing hemodialysis had lower titers that peaked at day 58 (276 AU/ml [IQR, 83.4-526.0]. We detected a positive antibody level in only three transplant recipients at day 36. In hemodialysis patients, those younger than 75 years had a higher antibody response versus those older than 75 years and serum albumin and Kt/V were positively correlated with serological response (P< 0.043 and P<0.019, respectively); nonresponders to HBV vaccine had the lowest anti-SARS-CoV-2 antibody titers. Conclusions Our results suggest that the postvaccination humoral response is strongly inhibited by immunosuppressant therapy in kidney transplant recipients and is reduced by the uremic condition in patients undergoing hemodialysis.