*Purpose: The diverse factors affecting the vaccine induced neutralizing antibody response in solid organ transplant recipients and their immunity against CoV2 variants are needed to be well characterize to understand how we improve the vaccine efficacy.

*Methods: Anti-CoV2 receptor binding domain (RBD) plasma antibody response and their neutralization potency in 29 kidney and 22 heart transplant patients was determined with recombinant RBD protein binding ELISA and by calculating the 50% virus neutralization titer of the plasma antibody with ACE2-Hu-HeLa cell based pseudo virus neutralization assay against CoV2 wild type and delta variant.

*Results: We detected strong binding and protective neutralizing plasma antibody response in SOTR who were infected with CoV-2 either prior to or after the first dose of vaccine (n=8), who showed high median IC50 value > 10,000 against both the CoV2 wild type strain and the more transmissible delta variant. In contrast to this, the CoV2 uninfected and vaccinated SOTR ( naïve vaccinees, n=43) had considerably lower anti-RBD plasma antibody binding titers, and only 19% of this population possessed minimally protective neutralizing antibody titer (IC50 >50) against the wild type CoV2 strain, which further decreased to 10% against the delta variant. While IgG and IgA were dominant isotypes of anti-RBD antibody induced by the CoV2 vaccines and correlated significantly (r=0.84, p=<0.001) with the CoV2 neutralization. The COV2 uninfected SOTR vaccinees who were within 1.5 years from transplantation or African American ethnicity were less likely to have detectable vaccine induced neutralizing antibody responses than the other populations. In the naïve vaccinees, administration of corticosteroids in combination with calcineurin or mTOR inhibitors and antimetabolites also negatively affected the CoV2 antibody responses, while female and younger organ transplant recipients tended towards higher IgM and IgA titers. Kidney transplant recipients showed better IgG responses vs heart transplant recipients and elevated serum creatinine levels correlated with poorer antibody response to the vaccines in both kidney and heart transplant groups.

*Conclusions: These results suggest that in the absence of immunity due to CoV2 infection, vaccination in SOTRs induces much lower protective antibody levels than in healthy controls and identifies African-American ethnicity, less than 1.5 years post transplantation as additional risk factors that further exacerbate these effects. Poor kidney function negatively affected the vaccine induced antibody response.

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