BACKGROUND: Patients with microdeletion 22q11.2 syndrome (MDS) exhibit immunological defects, characterized by abnormalities in the development of the thymus, which plays a crucial role in T-cell maturation and immune response. As a result, these patients may have impaired adaptive immunity, with decreased responses to vaccination. METHODS: This was a prospective observational study. Vaccine serology (tetanus, diphtheria, Haemophilus influenzae type b for children <
5 years, measles, varicella, hepatitis A and B, and SARS-CoV-2) and immune parameters were assessed in MDS patients aged between 1 and 25 years followed in Geneva between February 2022 and April 2023. RESULTS: 41 MDS patients were included. The median age was 13 years old. Most of them reported recurrent otitis and bronchitis up to 10 years, and a mild COVID-19 disease in the past. Immunological work-up indicated normal immunoglobulin levels and lymphocyte counts for the majority. Most patients were well vaccinated for tetanus, diphtheria, Haemophilus influenzae type b and measles, but only half were fully vaccinated for hepatitis B, and SARS-CoV-2 and only a quarter for hepatitis A. 70% of the patients had received 3 doses of pneumococcal conjugate vaccine in infancy but only a minority an additional dose. While most of them were seroprotected against tetanus, diphtheria, and Hib, a substantial number lacked seroprotection against varicella, measles, hepatitis B, and pneumococcus. CONCLUSION: This study suggests that regular assessment of antibody levels for measles, hepatitis B, varicella, and pneumococcus, regardless of vaccination status should be encouraged in MDS patients, with reimmunization according to vaccine serology, to enhance vaccine immunity.