CD38 and COVID-19: However, patients with HMs demonstrate an impaired humoral response to both mRNA- and vector-based COVID-19 vaccines [1,13,17,18,19] and up to 50–60% [19] of HMs patients do not show any response to two vaccine doses at all [13,20,21,22,23,24]; particularly, active treatment [25] at the time-point of vaccination, lymphocytopenia [26] as well as—most emerging—prior B-cell depletion (e.g., antibodies directed against CD20 [1,22] or CD38 [21] or inhibitors of Bruton’s tyrosine kinase [BTKi] [18,19]) significantly mitigate humoral vaccine response rates.