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.

Inclusion Criteria:

• Recipient of AlloSCT from related or unrelated donor.
• Adults (men and women) ≥18 years up to and including 45 years of age for vaccination.
• Patients can be included regardless of prior HPV vaccination prior to transplantation

Exclusion Criteria:

• Severe thrombocytopenia (under 50 x 10^9) not allowing intramuscular injection
• Severe acute GvHD grade III-IV.
• Extensive chronic GvHD requiring treatment with prednisone doses above 0.7 mg/kg/day plus at least two other systemic treatments against GvHD (for example ruxolitinib or photopheresis).
• Prednisone doses above 1mg/kg/day at study start.
• Treatment with rituximab 6 months before start of vaccination. Doses given later (unusual) do not require exclusion.
• Treatment within 3 months before start of vaccination with iv or sc immunoglobulin.
• Pregnancy, pregnancy desire or active pregnancy planning during time vaccine is given and up to three months after last vaccine dose.
• Treatment with blood thinning medication contraindicating intramuscular injection
• Allergy against Gardasil 9

Individual participant data (IPD) from this clinical trial will not be publicly shared to protect participant privacy and confidentiality. However, researchers may contact the Principal Investigator to request access to de-identified data.