Booster Dose of SARS-CoV-2 mRNA Vaccine in Kidney Transplanted Patients Induces Wuhan-Hu-1 Specific Neutralizing Antibodies and T Cell Activation but Lower Response against Omicron Variant

Kidney transplanted recipients (KTR) are at high risk of severe SARS-CoV-2 infection dueto immunosuppressive therapy. Although several studies reported antibody production in KTRafter vaccination, data related to immunity to the Omicron (B.1.1.529) variant are sparse. Herein, weanalyzed anti-SARS-CoV-2 immune response in seven KTR and eight healthy controls after thesecond and third dose of the mRNA vaccine (BNT162b2). A significant increase in neutralizingantibody (nAb) titers were detected against pseudoviruses expressing the Wuhan-Hu-1 spike (S)protein after the third dose in both groups, although nAbs in KTR were lower than controls. nAbsagainst pseudoviruses expressing the Omicron S protein were low in both groups, with no increaseafter the 3rd dose in KTR. Reactivity of CD4+ T cells after boosting was observed when cells werechallenged with Wuhan-Hu-1 S peptides, while Omicron S peptides were less effective in bothgroups. IFN-? production was detected in KTR in response to ancestral S peptides, confirmingantigen-specific T cell activation. Our study demonstrates that the 3rd mRNA dose induces T cellresponse against Wuhan-Hu-1 spike peptides in KTR, and an increment in the humoral immunity.Instead, humoral and cellular immunity to Omicron variant immunogenic peptides were low inboth KTR and healthy vaccinated subjects.