Cellular immunity predominates over humoral immunity after the first dose of COVID-19 vaccines in solid organ transplant recipients

Knowledge on the vaccine-induced cellular and humoral immunity and on immunogenicity of vector-based and mRNA vaccines in solid organ transplant recipients is limited. Therefore, SARS-CoV-2 specific T-cells and antibodies were analyzed in 40 transplant recipients and 70 age-matched controls after the first dose of vector-based or mRNA vaccines. Plasmablasts and SARS-CoV-2 specific CD4 and CD8 T-cells were quantified using flow-cytometry. Specific antibodies were analyzed by ELISA and neutralization assay. SARS-CoV-2 specific antibodies and T-cells were induced in both groups with significantly lower levels in patients. While antibodies were detected in 80% of controls and 5.3% of patients, specific CD4 and/or CD8 T-cells were more frequently found in both controls (84.3%) and patients (23.7%). The two vaccine types showed notable differences, as IgG and neutralizing activity were more pronounced after mRNA vaccination (p<0.0001 each), whereas CD4 and CD8 T-cell levels were higher after vector vaccination (p=0.009; p<0.0001). Plasmablast numbers were significantly higher in controls and correlated with SARS-CoV-2 specific IgG- and CD4 T-cell levels. In conclusion, assessment of antibodies is not sufficient to identify COVID-19-vaccine responders. Together with differences in immunogenicity among vaccines, this necessitates combined analysis of humoral and cellular immunity to reliably assess responders among immunocompetent and immunocompromised individuals.