A small pilot study from Thailand suggests that adolescents who undergo hematopoietic stem cell transplantation may develop a strong and persistent antibody response after receiving two doses of the human papillomavirus vaccine, challenging the assumption that three doses are always necessary for this medically vulnerable group. The findings, published in BMC Infectious Diseases, offer early evidence that a two-dose schedule can produce detectable antibodies against HPV types 16 and 18 in most young transplant recipients, while also maintaining measurable protection for years after vaccination. The results could eventually help doctors refine vaccination schedules for children and teenagers whose immune systems have been rebuilt after intensive cancer or blood-disorder treatment.
Hematopoietic stem cell transplantation, or HSCT, is used to replace diseased or damaged blood-forming cells with healthy stem cells. Before the transplant, patients typically receive chemotherapy, total-body irradiation, or both to destroy malignant cells and suppress the immune system so that the donated cells can take hold. This conditioning process also eliminates many of the immune cells and antibody-producing memory cells generated by previous infections or vaccinations. As a result, transplant recipients are often treated as though they have lost much of their earlier vaccine protection and must be revaccinated according to specialized schedules.
That issue is particularly important for HPV, a group of viruses transmitted mainly through sexual contact and responsible for nearly all cases of cervical cancer as well as a substantial proportion of anal, penile, vulvar, vaginal, and oropharyngeal cancers. HPV vaccines work best when administered before exposure to the virus, which is why routine vaccination focuses on adolescents. In healthy young people, the World Health Organization now recommends a single dose in many settings. Immunocompromised individuals, however, are generally advised to receive three doses because their immune systems may produce a weaker or less durable response. Whether adolescents recovering from HSCT truly need that full three-dose schedule has remained uncertain.
The research team at Ramathibodi Hospital and Mahidol University in Bangkok investigated the question through two complementary cohorts. The first included 23 adolescents between 9 and 15 years old who were at least one year beyond transplantation and had stopped taking immunosuppressive medicines for at least six months. Each participant received a quadrivalent HPV vaccine, which targets HPV types 6, 11, 16, and 18, at the start of the study and again six months later. Blood samples were collected six months after the first dose and one month after the second dose to determine whether the participants had developed antibodies against HPV16 and HPV18, two high-risk types strongly associated with cancer.
The scientists measured antibodies directed against the L1 capsid proteins of HPV16 and HPV18. L1 is the major structural protein forming the outer shell of the virus, and vaccine particles made from virus-like assemblies of L1 train the immune system to recognize the pathogen without containing viral genetic material. The investigators used an enzyme-linked immunosorbent assay, or ELISA, to detect immunoglobulin G antibodies in blood serum. In practical terms, seroconversion meant that a participant’s antibody index crossed the study’s predefined threshold for a positive result, indicating that vaccination had generated a measurable immune response.
Six months after the first dose, the response was incomplete but substantial: 54.5 percent of participants had seroconverted against HPV16, while 68.2 percent had seroconverted against HPV18. The second injection produced a striking increase. One month after the booster, 95.5 percent of adolescents had detectable antibodies against HPV16 and 90.9 percent had detectable antibodies against HPV18. The pattern suggests that the first dose primed the immune system in many participants, while the second dose activated immunological memory and generated a much broader antibody response. This boost is biologically important because antibodies can bind viral particles before they infect epithelial cells, blocking the earliest step in HPV transmission.
The vaccination was also well tolerated. Local reactions, such as pain, redness, or swelling at the injection site, were reported by between 22 and 39 percent of participants during the first 48 hours. Systemic reactions were uncommon, and the researchers observed no serious adverse events. For transplant recipients, who may face complications including graft-versus-host disease, recurrent disease, and infections, the absence of serious vaccine-related problems is reassuring. Nevertheless, the safety findings must be interpreted cautiously because the cohort contained only 23 participants and was not large enough to detect rare complications.
The study’s second cohort examined whether antibodies remained detectable over time. Fifteen vaccinated adolescents were assessed at intervals ranging from six months to 62 months after their final HPV vaccine dose. Every participant had detectable antibodies against HPV16, and all but one had detectable antibodies against HPV18. The cross-sectional design means that the researchers did not repeatedly follow the same individuals from vaccination onward, so the data cannot establish precisely how antibody concentrations changed in each participant. Even so, the results provide an encouraging snapshot of immune durability extending for more than five years in some adolescents.
The authors emphasize that the findings do not yet justify replacing existing recommendations for all HSCT recipients. The study was a pilot investigation with small numbers, no unvaccinated comparison group, and antibody measurements focused on only two HPV types. Detectable antibodies are an important marker of protection, but the relationship between a laboratory antibody index and real-world prevention of HPV infection is not perfectly defined. The participants were also vaccinated after recovering sufficiently from transplantation, and the results may not apply to patients who remain on immunosuppressive therapy or who experience severe immune complications. Larger studies should track antibody titers serially, assess responses to all vaccine-covered HPV types, and compare two-dose and three-dose schedules directly.
Despite those limitations, the study addresses a growing clinical challenge: how to restore vaccine protection efficiently without subjecting transplant survivors to unnecessary injections. Its results indicate that a two-dose quadrivalent schedule may be capable of producing near-universal antibody responses against HPV16 and HPV18 in selected adolescents once immune recovery has occurred. The evidence is not a final answer, but it supplies a valuable clue for future trials and could help guide individualized vaccination strategies. For young people who have already endured intensive treatment to rebuild their blood and immune systems, preserving protection against a preventable family of cancer-causing viruses may become an increasingly important part of long-term survivorship care.
