Breakthrough Varicella During Chemotherapy in Two Vaccinated Children With Acute Leukemia.

Breakthrough varicella (BTV), defined as varicella occurring more than 42 days after vaccination, is typically milder than primary varicella infection. However, its clinical course in immunocompromised children remains incompletely characterized. We report two pediatric patients undergoing chemotherapy for acute leukemia who developed BTV despite prior two-dose varicella vaccination. Both patients initially presented with atypical and sparse skin eruptions, which delayed diagnosis. During chemotherapy of profound immunosuppression, the rashes rapidly progressed, accompanied by fever and neutropenia. Rapid antigen testing confirmed varicella-zoster virus infection, and early treatment with intravenous acyclovir resulted in clinical improvement without visceral complications or nosocomial transmission. These cases highlight that BTV in immunocompromised hosts might present atypically yet deteriorate quickly. Prompt recognition and early antiviral therapy are essential to prevent severe outcomes. Clinicians should consider varicella even when only a few non-vesicular lesions are observed in vaccinated children receiving chemotherapy.