Background: Varicella can lead to severe complications in immunocompromised children, including those undergoing hematopoietic stem cell transplantation (HSCT) or chemotherapy. Preventing primary varicella zoster virus (VZV) infection is crucial in these populations to mitigate morbidity and mortality. This study aimed to evaluate the immunogenicity and safety of the live attenuated MAV/06 varicella vaccine in pediatric patients post-HSCT and post-chemotherapy. Additionally, it sought to compare fluorescent-antibody-to-membrane-antigen (FAMA) and enzyme-linked immunosorbent assay (ELISA) titers to establish effective cut-off levels for protection against varicella. Methods: The FAMA assay was conducted at the Vaccine Bio Research Institute, and a VARICELLA-ZOSTER ELISA (Vircell, Granada, Spain) kit, which relies on lysate from whole cells infected with VZV, was used to determine VZV IgG. A prospective cohort study was conducted with 76 pediatric patients under 18 years old who tested negative for VZV IgG via ELISA. Patients post-HSCT and post-chemotherapy were included. Participants received the MAV/06 varicella vaccine, and serologic responses were evaluated using ELISA and FAMA. Results: The median age of participants was 9.8 years, with acute lymphoid leukemia and acute myeloid leukemia being the most common underlying disease. Post-dose 1, the seropositive rate was 56.1% by ELISA and 97.2% by FAMA. Based on the FAMA seropositive cut-off ≥1:4, post-dose 1 geometric mean titers (GMTs) of seropositive patients in the post-HSCT group were 14.7 (95% CI, 11.3–19.1) versus 20.2 (95% CI, 13.0–31.3) in the post-chemotherapy group (p = 0.690). Based on a FAMA seropositive cut-off ≥1:16, the post-dose 1 GMT of patients considered seropositive in the post-HSCT group was 19.3 (95% CI, 15.6–24.0) versus 34.1 (95% CI, 21.0–55.4) in the post-chemotherapy group (p = 0.116), and post-dose 2 FAMA titers of 76.1 (95% CI, 14.6–398.1) in the post-HSCT group and 64.0 (95% CI, 11.4–358.1) in the post-HSCT group (p = 0.853) were observed. In patients with lower baseline FAMA titers (1:4 to 1:8), 66.7% in the post-HSCT group and 71.5% in the post-chemotherapy group achieved a greater than four-fold increase in FAMA titers post-dose 1, while those with higher baseline titers (≥1:16) did not. There were no serious adverse events or vaccine-related rashes occurring in any of the patients. Conclusion: The MAV/06 varicella vaccine is immunogenic in pediatric patients post-HSCT and post-chemotherapy, particularly when administered in a two-dose schedule using a cut-off FAMA titer of <1:16.
