Vaccination in patients with a hematologic condition Challenging the dogmas in vaccination of the immunocompromised

Patients with hematologic conditions are often immunocompromised due to the disease or the therapy they receive. This makes them prone to severe infections and infection-related mortality. Some infections can be prevented through vaccination. However, a key question remains: how effective are these vaccines in this immunocompromised population? In this thesis, I will explore vaccine immunogenicity in patients with hematologic conditions and seek to understand how their immunodeficiencies influence vaccine-induced immune responses. In part I of this thesis, humoral immune responses obtained after pneumococcal polysaccharide and conjugated vaccination are discussed in patients after allogeneic hematopoietic stem cell transplantation and patients with chronic lymphocytic leukemia. In part II, this thesis focused on the humoral and cellular immune response after SARS-CoV-2 mRNA vaccination in seventeen cohorts of patients with different hematologic conditions and treatments. We demonstrated that vaccination of these immunocompromised individuals is more effective than initially expected, inducing potent cellular and humoral immunity in the majority of patients, provided that multi-dose vaccination schedules are followed. This was observed both for mRNA vaccines and for conjugated vaccines. In case of high disease prevalence, for example in flu season or during a pandemic, inactivated vaccines should not be postponed in these patients, irrespective of treatment status, because it is more important to develop as much protection as possible, instead of achieving a protection rate comparable to that of healthy individuals. Future research should focus on both humoral and cellular immune responses after vaccination and always investigate the effect of booster vaccination(s) in this population.