Anti-S-IgG titers decreased in donors and HVs, whereas they remained stable in recipients, although at a significantly lower level. they remained stable in recipients, although at a significantly CR2 lower level. Indie negative factors associated with anti-S-IgG titers in recipients were age >60 years and lymphocytopenia (odds percentage: 2.35 and 2.44, respectively). == Conclusions == Kidney transplant recipients demonstrate delayed and attenuated reactions, with lower SARS-CoV-2 antibody titers after the second dose of the mRNA-based COVID-19 vaccine. The COVID-19 pandemic continues, driven by numerous variants, although implementation of vaccines against the SARS-CoV-2 is now common globally. Solid organ transplant recipients DMNQ (SOTRs) are particularly vulnerable because of the reduced response to vaccination[1]. Several articles possess reported the antibody response to the second SARS-CoV-2 vaccine in kidney transplant recipients compared with healthy individuals[2],[3],[4],[5]. However, the accurate recognition of factors traveling effective COVID-19 immunization remains lacking, which impedes the development of rational strategies for COVID-19 vaccination, particularly in SOTRs. Moreover, the toughness of the SARS-CoV-2 antibody postvaccination declined over a 3-month observation period in the general population[6], but antibody longevity in SOTRs is definitely unclear. Furthermore, no studies have tracked antibody titers of recipients after the second dose of the vaccine compared with individuals with chronic kidney disease (CKD) not receiving immunosuppressive medicines and healthy participants. Here, we performed a prospective observational study to investigate the kinetics and durability of the antibody titer in kidney transplant recipients compared with those in kidney donors and healthy settings. We also recognized factors negatively associated with the effectiveness of the SARS-CoV-2 vaccine in kidney transplant recipients. == Material and Methods == This study was performed in 2 parts. The 1st was a prospective cohort study, in which the anti-spike glycoprotein (S) immunoglobulin G (IgG) titers after a second COVID-19 mRNA vaccine dose and its switch over time were compared among kidney transplant recipients (recipient group), live kidney donors (donor group), and healthy volunteers (HV group). In the second part of the study, we analyzed factors associated with a failure to acquire a strong-positive anti-S-IgG response after a second vaccine dose in the recipient group. This study was authorized by the institutional review table of Sapporo City General Hospital (approval quantity: R02-060-802). All participants provided written educated consent. This study protocol was consistent with the honest recommendations of the Declaration of Helsinki. == Participants == Three hundred and seventy-eight adult individuals who underwent kidney transplantation at our hospital, 990 health care workers as HV settings, and 102 live kidney donors as CKD settings were enrolled in this study. The exclusion criteria were a DMNQ history of COVID-19, positive anti-S-IgG and/or antinucleocapsid protein (N) IgG results before the 1st vaccination, and a positive anti-N IgG result after the second dose of vaccine, which indicates asymptomatic illness with COVID-19 during the study period. Additionally, participants who tested bad for anti-S-IgG after the second vaccine dose were not adopted up with this study. Participants received 2 doses of the BNT162b2 or mRNA-1273 vaccine, with an interval of 3 or 4 4 weeks, respectively. Anti-S and anti-N IgG were tested 1 to 6 months before the 1st vaccination and re-evaluated 4 to 12 months after the second dose of the vaccine, using the Elecsys Anti-SARS-CoV-2 S RUO (Covas 8000/e 801; Roche Diagnostics, Mlan, France). == Evaluation of Positivity Rate for and Durability of Anti-S-IgG Response After Second Vaccine Dose == To determine the acquisition of immunity after the vaccine, anti-S-IgG with <0.8 U/mL, 0.8 U/mL up to 15 U/mL, and 15 U/mL were defined as negative, weakly positive, and strongly positive, respectively, whereas anti-N-IgG was negative. Relating to data provided by the manufacturer of the Elecsys Anti-SARS-CoV-2 S DMNQ RUO (Covas 8000/e 801; Roche Diagnostics), anti-S-IgG with 0.8 U/mL was defined as positive, and values 15 U/mL, in particular, were correlated with the presence of neutralizing antibodies, having a positive predictive value of 99.1%. To investigate the durability of the anti-S-IgG response, we performed follow-up checks every 4 to 8 weeks until 24 weeks after participants were confirmed positive for anti-S-IgG. == Immunosuppression == All kidney transplant recipients received maintenance DMNQ immunosuppressive therapy that consisted of calcineurin inhibitors (CNI) and mycophenolate mofetil (MMF), with or without methylprednisolone (MP) or everolimus (EVR). Briefly, our induction.