BACKGROUND: Limited data are available regarding the development and durability of immune responses following COVID-19 infection or vaccination in pediatric solid-organ transplant (SOT) recipients. METHODS: Renal, liver, or intestinal transplant recipients <
21 years of age followed at Seattle Children's Hospital were enrolled from August 2020 to May 2021. Blood samples were collected at ~6-month intervals for up to 3 years and tested for antinucleocapsid (N) antibodies. COVID-19 vaccination data were collected from the Washington State Immunization Information System and/or the medical record. Semi-quantitative anti-S IgG testing was performed on all postvaccine samples using the Abbott Architect platform. We further evaluated a subset of postvaccine samples using variant-specific quantitative binding (Meso Scale Discovery, MSD) immunoassays and pseudovirus-neutralization assays. Antibody levels were compared over time and by vaccine category. RESULTS: We followed 83 SOT recipients for a median of 12.5 months (IQR 7.0, 28.3). Overall, 16 (19.3%) participants had evidence of SARS-CoV-2 infection based on anti-N antibody detection. Forty-six (55%) participants had a blood sample collected >
14 days after receipt of a vaccination. Serum IgG to spike antigens (anti-S antibody) increased following vaccination and increased with the number of vaccine doses received as assessed by both the Abbott and MSD assays. Neutralizing activity was significantly lower against the Omicron subvariants compared to the ancestral strain. CONCLUSION: Pediatric SOT recipients demonstrated strong antibody responses following SARS-CoV-2 vaccination, with higher anti-S antibody responses following >
2 doses of vaccine. Our study offers unique longitudinal immune response data in this vulnerable patient population.