Here, we report a case of a probable reactivation of SARS-CoV-2 in an immunosuppressed patient who previously had COVID-19 and who had received one dose of COVID-19 mRNA vaccination (BNT162b2, BioNTech/Pfizer). A 61-year-old male suffering from acute myeloid leukemia (AML) was admitted to our center for a planned allogeneic hematopoietic stem cell transplantation (allo-HSCT). AML was diagnosed in March 2020. The patient received no intensive induction chemotherapy because of coronary heart disease as comorbidity. The patient received 11 cycles of the hypomethylating agent decitabine combined with the BCL2-inhibitor venetoclax until February 2021. The AML was morphological in complete remission but with molecular persistence of NPM1. Thus, allo-HSCT was planned. In February 2021, the patient had received the first dose of a COVID-19 mRNA vaccine (BNT162b2, BioNTech/Pfizer). Two weeks later, when the beginning of the 12th cycle of chemotherapy was planned, he had presented to another center with myalgia, fever, and gastro-intestinal symptoms, without respiratory symptoms. A naso-oropharyngeal swab was tested SARS-CoV-2 positive by PCR (Table 1). During his stay, the patient did not develop any significant respiratory symptoms and did not require oxygen supplementation. CT scan showed subpleural ground glass infiltration compatible with COVID-19 pneumonia with a very limited extension. He was treated with symptomatic therapy and standard antithrombotic prophylaxis. The clinical course was favorable and the patient was discharged after 11 days. Two following naso-oropharyngeal swabs, performed 7 and 10 days after diagnosis, respectively, were negative. At the same time, anti-SARS-CoV-2 IgG with a high titer of 743 AU/ml was detected; IgM was equivocal. Since the patient had developed the infection 2 weeks after receiving the first dose of the vaccine and since SARS-CoV-2 IgG antibodies were already positive, we decided not to administer the second dose of the COVID-19 vaccine, assuming that the patient would acquire natural immunity after the infection. Due to the good general condition of the patient and the molecular leukemia persistence, we decided to proceed to allo-HSCT without any delay. On admission at our center, the patient was asymptomatic and physical examination was normal. Laboratory tests showed mild anemia and thrombocytopenia with normal CRP and without signs of a coagulopathy. The AML was in morphological complete remission prior to allo-HSCT. A CT showed no residual signs for pulmonary infection. SARS-CoV-2 was undetectable in the naso-oropharyngeal swab. We started the planned conditioning regimen with fludarabine, cyclophosphamide, and 2-Gy total body irradiation (TBI) which was well-tolerated. The GvHD prophylaxis consisted of high-dose cyclophosphamide post-transplant, tacrolimus, and mycophenolate mofetil. The stem cell source was bone marrow from his haplo-identical son. A few days after starting conditioning, the patient developed lymphopenia (Table 1) in addition to the already known anemia and thrombocytopenia. A new, routinely performed SARS-CoV-2 saliva PCR on day -4 before HSCT came out to be positive, with a semi-quantitative CT value of 35.9 (cycle threshold). The antibody testing was repeated, showing 136 U/ml anti-SARS-CoV-2 spike antibodies (normal range < 0.7 U/ml) and 31.9 U/ml anti-SARS-CoV-2 nucleocapsid-antibodies (normal range <= 1 COI). The antibody testing was performed with the electro-chemiluminescence immunoassay (ECLIA) method. The patient was afebrile and asymptomatic and the inflammation markers were normal. We continued the conditioning chemotherapy. Since a worsening of the immune situation was expected, we administered one unit of convalescent plasma. A confirmatory naso-pharyngeal swab PCR tested SARS-CoV-2 positive. The quantitative viral load was however below the limit of detection (< 1000 copies/ml). Two days later, a monitoring SARS-CoV-2 quantitative PCR test showed again a viral load of <1000 copies/ml and the isolation was ended according to the internal guidelines. In addition, the following SARS-CoV-2 quantitative PCR tests showed viral loads of < 1000 copies/ml (Table 1). The neutrophil engraftment (> 0.5 G/l) occurred on day +23 and the patient discharged on day +31. Lymphopenia persisted until discharge (0.02 G/l). Two months after allo-HSCT, the patient is still in complete molecular remission (NPM1A negative in peripheral blood and bone marrow) without signs of acute graft-versus-host disease.