*Purpose: We examined a positivity of corona virus antibody in kidney transplant recipients who received two-doses BNT162B2 (mRNA) vaccination during the beginning 6 months in 2021.

*Methods: The subjects include 111 kidney transplant recipients who received two-doses BNT162B2 (mRNA) vaccination, consisting of 61 male and 50 females, 53.7±14.7 years old on average. Healthy 10 volunteers were also enrolled in this study as a control group. We adopted LABScreen COVID plus beads (One Lambda) as an antibody reagent to detect antibodies against corona viruses including 5 types structural proteins such as extracellular domain (ECD), Spike 1, Spike 2, receptor-binding domain (RBD) and nucleoside.

*Results: Among 111 recipients, 46 recipients showed positivity (41%), whilst all control showed positivity (100%). Among 100 recipients taking tacrolimus, 37 recipients showed positivity, although 9 recipients out of 10 taking cyclosporine showed positivity (90%). It is noteworthy that recipients receiving rituximab (CD20 antibody) significantly shows lower rate of antibody positivity. The period between vaccination and rituximab administration is closely related to the positivity of antibody positivity. In recipients without rituximab administration, lower trough level for tacrolimus and MMF is related to higher positivity of antibody production. Positivity pattern in 5 protein fragments shows individually various different in transplant recipients, while it shows stable pattern in healthy volunteers.

*Conclusions: Kidney transplant recipients showed lower positivity of corona virus antibody compared to healthy volunteers, which is deeply related to immunosuppression in the past. This assay in this study yields reproductive excellent results for corona virus antibody. The assay helps us to find the appropriate booster vaccination time and doses on the basis of these past results.

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