*Purpose: New recommendations suggest that measles-mumps-rubella vaccine (MMR-vx) and varicella vaccine (VZV-vx) may be administered to certain non-immune transplant recipients. The purpose of this study was to determine safety and immunogenicity of MMR and VZV-vx in a cohort of pediatric transplant recipients.

*Methods: Pediatric liver and kidney transplant recipients across the US received MMR and/or VZV-vx according to individual center guidelines. Demographic, clinical and laboratory data were collected pre- and post-vx.

*Results: 147 children (141 liver, 5 kidney, 1 liver-kidney recipient) from 13 centers received 295 doses of live vxs (155 VZV, 140 MMR). The median age at transplant and first post-transplant vx was 0.9 (IQR 0.6-1.8) and 8.3 (IQR 4.7-13.8) years. Immunosuppression at time of first post-transplant vx included tacrolimus (n=140, 97% with trough <8), cellcept (n=12), sirolimus (n=6), steroids (n=6), azathioprine (n=4), everolimus (n=2) or none (n=1). The median ALC was 2600/µL (IQR 1995-3185), serum IgG was 1028 mg/dL (IQR 857-1252) and CD4 was 917 cells/µL (IQR 704-1369). Pre-transplant, 40 children received MMR-vx and 40 received VZV-vx (23 received both). Of the children who received either MMR or VZV-vx pre-transplant and had subsequent pre-transplant antibodies (Abs) checked, 15/30 (50%) had positive VZV Abs, 5/7 (71%) had positive measles Abs, 3/5 (60%) had positive mumps Abs and 8/13 (62%) had positive rubella Abs. Of the 111 children who received at least one post-transplant VZV-vx, 42 of 52 (81%) had Abs checked and mounted a positive varicella IgG. Of the 103 children who received at least one post-transplant MMR-vx, Abs were checked and an IgG response mounted in 46 of 57 (81%) for measles, 31 of 38 (82%) for mumps and 31 of 38 (82%) for rubella. At one-year post-vx, 7/9 (78%) children with follow-up labs had positive VZV IgG, 8/10 (80%) had positive rubeola IgG, 9/9 (100%) had positive rubella IgG and 8/9 (89%) had positive mumps IgG. Following vx three children developed clinical varicella (all more than 10 days post vx) that resolved within a week of starting oral or intravenous antiviral therapy. No child developed measles. No child developed acute cellular rejection within the first month after vx.

*Conclusions: Live vxs can be safe and immunogenic in certain transplant recipients. Further studies are ongoing to correlate demographics and immunosuppression with Ab response. Similarly, longitudinal assessment is ongoing to understand ideal timing of vaccination, length that immunity persists, and need for boosters.

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