LAUSR

Background: The majority of solid organ transplant recipients did not develop an appreciable serologic response following two doses of mRNA SARS-CoV-2 vaccine. Methods: We analyzed the humoral response following third dose of BNT162b2 vaccine in 130 kidney transplant recipients, compare to 48 healthcare workers, and associated factors, including pre-vaccine cellular immune response, by evaluating intracellular cytokine production following stimulation of donor's peripheral blood mononuclear cells. Results: Following two doses, most of controls (47/48, 98%) and only 40% (52 of 130) kidney recipients were seropositive (p<0.001). Most seronegative recipients developed a serologic response after the booster (47 out 78, 60%), thus bringing the total number of seropositive recipients to 99/130 (76%). Following the third dose, there was a significant increase in antibodies titers in both groups. Decreased humoral response was significantly associated with an older age, lower lymphocyte count, and a lower level of antibodies before booster administration. CD4+TNFα+ and CD4+INFγ+ were correlated with mean increase in antibody titers. Conclusions: A third dose of BNT162b2 mRNA vaccine in kidney recipients is safe, and effectively results in increased IgG anti-S levels, including in individuals who were seronegative after two doses. Long-term studies of the length of the immune response and protection are required.