Vaccine-related influenza virus B infection in a child with an undiagnosed B-cell acute lymphoblastic leukemia.

We report a case of Influenza type B (lineage Yamagata) infection in a child who received the live attenuated influenza virus vaccine before being diagnosed with B-cell acute lymphoblastic leukemia. The patient developed a mild disease that persisted for 18 days and resolved without antiviral treatment. The prolonged infection of an attenuated virus in an immunocompromised host might pose a risk of reversion or evolution to a more pathogenic strain. Prompt prevention, identification, and monitoring of similar cases are desirable to avoid the development of severe illness, which could complicate patient management.