*Purpose: Solid-organ transplant recipients are at increased risk of developing human papillomavirus (HPV)-related diseases. The immunogenicity of HPV vaccine for solid-organ transplant population needs further investigations. To provide evidence of immunogenicity of the quadrivalent HPV vaccine for pediatric kidney and liver transplant recipients.

*Methods: To evaluate the immunogenicity of a quadrivalent HPV vaccine, a prospective observational study included females aged 12-19 years who had received renal or liver transplant as well as healthy females. Gardasil®, the quadrivalent HPV recombinant vaccine, was administered to study participants intramuscularly at 0, 2, and 6 months. Serum antibodies to HPV types 6, 11, 16, and 18 antibodies were measured using a competitive Luminex immunoassay (reported in milli-Merck Units/mL; mMU/mL).

*Results: The study included 17 transplant recipients (7 kidney and 10 liver transplant recipients) as well as 20 control participants. The seropositivity rates for HPV types 6, 11, 16, and 18 in post-vaccine samples among the transplant recipients were 76.5%, 82.4%, 82.4% and 82.4%, respectively, while all control participants achieved 100% seropositivity for all 4 HPV types. The geometric mean tires were 44.0, 14.8, 57.1 and 64.8 mMU/mL for kidney transplants, 379.3, 339.8, 1,431.0 and 452.4 mMU/mL for liver transplants, and 364.7, 238.5, 889.8 and 228.0 mMU/mL for healthy participants, respectively. Liver transplant recipients had significantly higher titres than kidney transplant recipients for HPV type 6 and 11 (p=0.025 and 0.008, respectively).

*Conclusions: Among the transplant population studies, the overall immunogenicity of HPV vaccine is satisfactory (more than 70% seroconversion). The results support the recommendation for use of HPV vaccination in pediatric transplant recipients given the significant risk of HPV-related diseases in the population.

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