Patients who undergo allogeneic stem cell transplantation lose previously acquired immunity and are routinely revaccinated against many infectious diseases. For several reasons, these patients have a long-term immune deficiency due to the transplant itself (lack of immune reconstitution) and due to possible complications, primarily GvHD and its treatment. The risk of secondary malignancy in the long-term following an allogeneic bone marrow transplant is greatly increased, and secondary cancer cases account for a significant proportion of late deaths in both women and men after transplantation. Some of these secondary cancers are associated with HPV. The risk of cervical cancer has been reported to be 13 times increased compared to a healthy population. Therefore in this trial, the aim is to study immune response (antigen-specific antibody response) after vaccination with 9-valent HPV vaccine (Gardasil 9®) in adult women and men (up to and including 45 years of age) who have undergone allogeneic stem cell transplantation. In this trial, the sponsor will compare early (start 9 months after tx) with late (start 15 months after tx) vaccination.