The purpose of this clinical trial is to investigate the efficacy of the EBV mRNA vaccine (WGc-0401 injection) in preventing EBV-related diseases after allogeneic hematopoietic stem cell transplantation (allo-HSCT), and to evaluate the safety and efficacy of this vaccine in patients following allo-HSCT.

The main study questions are:

1. The incidence of grade III-IV acute graft-versus-host disease (aGVHD) within 100 days, and the occurrence of ≥ grade 3 adverse events (AEs) that are possibly or definitely related to the vaccine, in patients receiving EBV mRNA vaccination after allo-HSCT.
2. To determine the optimal biological dose (OBD) of the EBV mRNA vaccine in patients after allo-HSCT among the dose levels of 25μg, 50μg, 75μg, or 100μg.
3. EBV-ELISpot levels, the incidence of EBV viremia (EBV DNAemia), the incidence of post-transplant lymphoproliferative disorders (PTLD), disease relapse rate during follow-up, non-relapse mortality (NRM), and immunogenicity indicators (IFN-γ+ T cells, immune cell analysis, cytokine profiles, EBV glycoprotein antigen antibodies).

Participants will:

1. Receive three intramuscular injections of the EBV mRNA vaccine on days 30, 44, and 81 after allogeneic hematopoietic stem cell transplantation (d30, d44, d81).
2. Be hospitalized for at least 72 hours after each vaccination for close monitoring (with a focus on CRS and aGVHD), and undergo intensive safety assessments throughout the dose-escalation period (at least 28 days).
3. Return for clinical visits on days 7 (d37, d51, d88), day 14 (d58), and day 180 (d180) after vaccination for EBV-ELISpot, EBV-DNA quantification, and immunogenicity testing, with continued long-term follow-up to evaluate safety and the persistence of vaccine-induced immune responses.

Inclusion Criteria:

• Age ≥14 years, regardless of gender;
• Patients undergoing allogeneic hematopoietic stem cell transplantation;
• Voluntary participation in the clinical study and signing of the informed consent form.

Exclusion Criteria:

• History of vaccine allergy;
• Presence of active acute graft-versus-host disease (aGVHD) at screening;
• Severe impairment of cardiac, hepatic, or renal function;
• Body temperature >38°C within 72 hours prior to enrollment;
• Inability to communicate reliably with the investigator or unlikely to comply with study requirements;
• Any other condition deemed by the investigator to make the patient unsuitable for enrollment.

Describe which specific IPD will be shared.