(1) Background: Children and young adults with cancer are poorly represented in COVID-19 vaccination studies, and long-term protection conferred by vaccination is not known. (2) Objectives: 1. To determine the adverse effects associated with BNT162B2 vaccination in children and young adults with cancer. 2. To assess its efficacy in stimulating immunological response and in preventing severe COVID-19 disease. (3) Methods: Retrospective single-center study evaluating patients aged 8 to 22 years, with cancer, who underwent vaccination from January 2021 to June 2022. ELISA serologies and serum neutralization were collected monthly from the first injection. Serologies below 26 were considered negative, while those above 264 BAU/mL were considered positive and indicative of protection. Antibodies titers were considered positive above 20. Data on adverse events and infections were collected. (4) Results: 38 patients were included (M/F = 1.7, median age 16 years), of whom 63% had a localized tumor and 76% were undergoing treatment at the time of the first vaccination. Two or three vaccine injections were administered in 90% of patients. Adverse events were mainly systemic and not severe, except for seven grade 3 toxicities. Four cancer-related deaths were reported. Median serology was negative the month following the first vaccination and became protective during the third month. At 3 and 12 months, median serology was 1778 and 6437 BAU/mL, respectively. Serum neutralization was positive in 97% of the patients. COVID-19 infection occurred despite vaccination in 18%; all were mild forms. (5) Conclusions: In children and young adults with cancer, vaccination was well tolerated and conferred effective serum neutralization. COVID-19 infections were mild, and vaccine seroconversion persisted after 12 months in most patients. The value of additional vaccination should be further established.

LETTER TO THE EDITOR experiments and the patients and their family for participation in the study. CONFLICT OF INTERESTCaroline Donze, Laetitia Ninove, and Xavier de Lamballerie have no conflict of interest to disclose. Paul Saultier reports receiving support for attending scientific meetings and educational events from Baxalta-Shire, Novo Nordisk, Pfizer, Octapharma, Roche, Servier, and Sobi and has received honoraria (consultant on advisory boards) from Octapharma outside the area of the submitted work. Nicolas Andre reports receiving grants and/or drugs for trials from Bristol Myers Squibb and Pierre Fabre, outside the area of submitted work; receiving travel support from Bristol Myers Squibb; and participating as a scientific advisory board member (without receiving personal fees) for Bayer and Bristol Myers Squibb, and Partners Therapeutics, outside the area of the submitted work. AUTHOR CONTRIBUTIONS Caroline Donze and Paul Saultier wrote the paper. Laetitia Ninove and Xavier de Lamballerie supervised and interpreted the biological analyses. Caroline Donze, Paul Saultier, and Nicolas Andre collected clinical data. Xavier de Lamballerie, Paul Saultier, and NicolasAndre coordinated the study. All authors edited and approved the final manuscript as submitted and agree to be accountable for all aspects of the work.