Immunogenicity and Safety of Two Respiratory Syncytial Virus Vaccine Strategies in Lung and Allogeneic Haematopoietic Stem Cell Transplant Recipients.

Respiratory syncytial virus (RSV) is a major cause of acute respiratory infections (ARIs), which occur in seasonal outbreaks. Among the adult population, immunocompromised individuals, particularly lung transplant recipients and recipients of allogeneic haematopoietic stem cell transplants (HSCTs), are at the highest risk of severe ARI. RSV can also cause prolonged inflammation of the airways, leading to persistent deterioration in lung function. Thus, in lung transplant recipients, RSV is a major cause of chronic lung allograft dysfunction (CLAD), the leading cause of lung transplant failure and mortality beyond the first post-transplant year. In recipients of ACSH, RSV infection is associated with the onset and/or worsening of bronchiolitis obliterans syndrome, which leads to increased mortality. Preventing RSV-associated lower respiratory tract disease (LRTD) and its short- and long-term consequences in these populations is therefore a key priority. Three inactivated RSV vaccines, targeting the RSV fusion glycoprotein in its pre-fusion conformation (RSVpreF), have recently been approved for use in adults. These comprise two recombinant protein vaccines - one with an adjuvant (RSVpreF3-AS01, Arexvy®, GSK) and one without (RSVpreF, Abrysvo®, Pfizer) - and one mRNA vaccine. Although these vaccines have demonstrated their safety, immunogenicity and efficacy (reducing the risk of RSV-related IMRI by approximately 80-85 per cent) in adults aged 60 and over, the pivotal trials excluded immunocompromised patients. To date, there is limited data on the efficacy, immunogenicity and tolerability of these vaccines in high-risk immunocompromised populations, such as recipients of bone marrow transplants and allogeneic stem cell transplants. Real-world studies suggest that RSV vaccines may reduce the risk of RSV-associated acute respiratory infection and hospitalisations in elderly immunocompromised adults. With regard to immunogenicity, the available cohort studies, conducted on small sample sizes, indicate that the vaccine response is heterogeneous and, overall, lower than that observed in immunocompetent older people. Preliminary data suggest that booster vaccination regimens may be necessary to optimise the immune response in these populations. Given the significant morbidity associated with RSV in recipients of bone marrow transplants and allogeneic stem cell transplants, it is a priority to assess the immunogenicity and safety profile of RSV vaccines, and to determine the optimal vaccination schedule for these high-risk, immunocompromised populations. The investigators hypothesise that the immune response to RSV vaccines in severely immunocompromised TP and ACSH recipients is weaker than in immunocompetent older adults, and that enhanced vaccination strategies may be required. In this study, two vaccination strategies will be evaluated in these populations: * a single dose of the adjuvanted RSVpreF3-AS01 vaccine (Arexvy®), * or a booster regimen comprising two doses of the non-adjuvanted RSVpreF vaccine (Abrysvo®), administered one month apart.

Inclusion Criteria: * Prior lung transplantation (including combined transplants) performed ≥3 months before the first vaccine administration (LTR) OR prior allogeneic haematopoietic stem cell transplantation performed within ≥3 months to 5 years before the first vaccine administration (HSCTR); * Active follow-up at one of the study centres; * Age ≥18 years at inclusion; * Providing written, informed consent. Exclusion Criteria: * Previous vaccination with any licensed or investigational RSV vaccine; * History of any reaction or hypersensitivity likely to be exacerbated by any component of the vaccines; * Active malignant disease at inclusion (with the exclusion of non-melanoma skin cancer); * Receipt of immunoglobulins and/or any plasma derivatives during the period starting 90 days before the first study vaccine administration, or planned administration during the study period; * Acute cellular graft rejection episode treated by corticosteroid boli within one month before first vaccine administration (LTR); * Ongoing plasmapheresis for acute humoral graft rejection (LTR); * Ongoing, acute graft-versus-host disease of grade II/III/IV (HSCTR); * Chronic graft-versus-host disease which is not stable for at least one month prior to first vaccine administration (HSCTR); * Anti-CD20 and/or anti-CD52 therapy within the last 6 months; * Administration of anti-thymocyte globulin within the last 3 months; * Any vaccine administration within two weeks before inclusion or planned receipt within two weeks following each experimental vaccine administration; * Known bleeding disorder which, in the opinion of the investigator, contraindicates intramuscular injection; * Any female participant who is pregnant, lactating or planning to become pregnant during the study period. * Expected unavailability for the planned study visits; * Concurrent participation in another active clinical study at the time of inclusion which includes the application of investigational products (medication, vaccine) or an interventional research using an invasive medical device during the study period; * Acute, severe febrile illness at the time of inclusion; * Any person deprived of liberty by a judicial or administrative decision; * Any person under legal protection measures; * Any person non affiliated to National French social security system or equivalent.