*Purpose: Transplant recipients experience lower seroconversion rates after COVID-19 vaccination compared with healthy adults and have been prioritized to receive booster doses. To assess pre- and post-booster cellular and antibody immunity after COVID-19 vaccination in adolescent and young adults with liver transplants.

*Methods: In this preliminary report, we measured pre-booster frequencies of peripheral blood T-and B-cells which expressed the inflammatory marker CD154 after overnight stimulation with peptide mixtures representing the spike protein S, its S2 component which is conserved between SARS-CoV-2 and human coronaviruses, and the S1 component, which is specific to SARS-CoV-2, which also contains its receptor binding domain (RBD). Serum from each sample was assayed for anti-RBD and anti-S IgG with ELISA. OD at 450nm of 0.45 or greater implied presence of either antibody. Frequencies of monocytic and polymorphonuclear (PMN) myeloid-derived suppressor cells (M-MDSC, PMN-MDSC) were also measured with flow cytometry.

*Results: Seven LT recipients, three of whom also received intestine allografts included 6 Caucasian and one Asian. Two received mRNA1273 and 5 received BNT162b vaccination. Median age was 15 years (range 13-20) male:female distribution was 3:4. Subjects were first sampled at median 14.5years (range 10.7-18) from transplant. Median tacrolimus whole blood concentration was 4.6 ng/ml (range 2-8.4). All but one received two doses of vaccination; one opted to receive a single dose, and a liver-intestine recipient received a third dose after primary vaccination. Anti-S-IgG and anti-RBD-IgG antibodies were detected in 6/7 subjects. S2-reactive and S-reactive T-cells were detected in five and B-cells were detected in 6/7 (Figure 1). Mean+/-SEM frequencies (%) of S2 and S-reactive cells were 1.8±0.7 and 1.3±0.7 for CD4, 1.8±0.8 and 1.7±0.8 for CD8 and 7.4±1.9 and 6.2±2.4 for CD19+B-cells. In the liver-intestine recipient with no antibody response to primary vaccination, S2 and S-reactive T-cells were undetectable (Figure 1) and tacrolimus whole blood concentration of 8.4 ng/ml exceeded levels which were <6 ng/ml in the remaining subjects. A booster dose led to detectable antibodies.

*Conclusions: Most adolescent transplant recipient with a liver-inclusive allograft show robust antibody and cellular response, when vaccinated with mRNA vaccines late after transplantation. Recipients who fail to seroconvert and show impaired T-cell responses may benefit from booster doses.

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