Vaccinations in Pediatric Hematology and Oncology: Biologic Basis, Clinical Applications, and Perspectives.

Children with hemato-oncological diseases represent a heterogeneous population at heightened risk for vaccine-preventable diseases. Their immunosuppressed state reduces vaccine efficacy and raises safety concerns regarding live attenuated vaccines due to the risk of viral reactivation. The immunological and clinical implications of the single conditions are significantly different; therefore, specific vaccination strategies are needed. Despite the availability of vaccine guidelines for immunocompromised patients, clinical practice remains highly variable. It is generally recommended to avoid vaccinations during chemotherapy, with some exceptions for influenza, pneumococcal, and, in some countries, hepatitis B vaccines. The timing of immune recovery after chemotherapy depends on the specific treatment and most guidelines recommend administering vaccines 3-6 months after treatment cessation. Concerning HSCT, the timing of immune recovery is affected by several factors such as the HSCT platform, graft-versus-host disease (GvHD), and infections. Inactivated vaccines are typically administered 3-6 months post-HSCT, while live attenuated vaccines are delayed for at least two years. In children with asplenia or hyposplenism, recommendations focus on immunization against encapsulated bacteria, with tailored schedules based on the patient's age and underlying condition. This paper explores the biological factors influencing vaccination efficacy and safety in pediatric hematology and oncology patients. It also provides an updated overview of the available evidence and current vaccination guidelines. Finally, this paper highlights the main clinical and research areas for further improvement to provide tailored vaccination schedules for this vulnerable population.