The investigators aim to perform a randomized clinical trial to determine the optimal timing of hepatitis B vaccination after hematopoietic cell transplantation (HCT) through evaluating the immunity effect of two different vaccination schedules (initiated at 3 or 6 months after transplantation) in patients with different immune reconstitution status.

Inclusion Criteria:

1. Patients must be ≥ 16 years old;
2. Patients receiving hematopoietic cell transplantation;
3. Patients achieving complete molecular remission;
4. Patients or their guardians have to sign an informed consent form before the start of the research procedure.

Exclusion Criteria:

1. Multiple transplantations;
2. Donors' HBV-DNA or HBsAg are positive;
3. Patients' HBV-DNA or HBsAg are positive before transplantation or < 3 months after transplantation;
4. Patients who are unable to comply with the research treatments and monitoring requirements due to mental or other medical conditions;
5. Patients who are ineligible for the study due to other reasons which would cause unacceptable risks to the patients.