*Purpose: Chronic immunosuppression can impair antibody responses after natural infection and vaccination in SOT recipients. It is not known whether antibody responses are impaired in SOT compared with non-SOT patients with COVID-19

*Methods: We evaluated IgG responses to the spike (S) and receptor binding domain (RBD) antigenic sequences of SARS-CoV-2 after COVID-19-infection in SOT and non-SOT patients using enzyme-linked immunosorbent assay (ELISA). The S protein consists of a conserved C-terminal and less conserved N-terminal S1 sequence which contains the RBD. An optical density (OD) at 490 nm of 0.45 or greater, or < 0.45 was read as positive and negative ELISA result based on pre-clinical validation in 148 subjects. S1-reactive, S2-reactive and S-reactive CD4 cells that expressed CD154 were measured with flow cytometry after overnight stimulation with respective peptide mixtures for these antigens.

*Results: 204 total study subjects, mean+/-SD age 47.5+/-21 years, were sampled at mean 12.6 (range 0-94) days after diagnosis. Subjects included 107 males, 74 SOT (liver-48, kidney-26) and 130 non-SOT. Among them, 103 patients with COVID-19 included 32 SOT and 71 non-SOT. Antibody measurements were performed in 74 COVID-19 patients. Fifty one of 74 patients received convalescent plasma. Anti-spike and anti-RBD IgG were present in 49 of 51 (96%) and 47 of 51 (92%) patients, respectively. Among the remaining 23 patients who did not receive plasma, anti-spike IgG and anti-RBD IgG were present in 21 (91%) and 16 (69.5%) patients, respectively. The incidence of anti-RBD IgG was significantly lower in SOT, 2 of 7 or 29%, compared with non-transplant patients, 14 of 16 or 88% (p=0.011). Between-group differences in the incidence of anti-spike IgG were not significant (5/7 or 71% SOT vs 16/16 or 100% SOT, p=NS). One of 23 patients, an SOT recipient died and showed no IgG to S or RBD antigens. Antibody titers reflected in OD490 readings were lower in SOT compared with non-SOT for anti-spike IgG (mean 2.2+/-0.6 vs 1.4+/-1.2, p=0.16, NS) and anti-RBD IgG (1.8+/-0.9 vs 0.54+/-0.7, p=0.004). Subjects without and with anti-RBD antibody did not differ in timing of the sample from diagnosis (mean+/-SD 18+/-12.5 vs 12+/-12, p=0.258, NS, respectively), frequencies of S-, S1 or S2-reactive T-cells (mean 3.1+/-2.4% vs 1.8+/2%, p=0.225, NS, respectively), or proportions of patients requiring intubation (2/7 or 29% vs 4/16 or 25%, p=1.00, NS, respectively).

*Conclusions: Chronically immunosuppressed liver and kidney transplant recipients demonstrated impaired antibody responses to SARS-CoV-2 spike antigens, especially to less conserved RBD-containing viral sequence. This finding may portend impaired vaccine efficacy in transplant recipients.

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