Aim. To analyze the characteristics of novel coronavirus infection COVID-19 in hairy cell leukemia (HCL) patients and to assess humoral immune response in those of them who had COVID-19 and/or were vaccinated against SARS-CoV-2.
 Materials & Methods. This paper documents the retrospective analysis of the data from 118 HCL patients followed-up at the National Research Center for Hematology from March 2020 to October 2021. Within this period, 99 out of 118 patients had novel coronavirus infection COVID-19; 17 out of 99 COVID-19 patients were vaccinated against SARS-CoV-2 (3 of them prior to being infected and 14 of them after that). The rest of 19 HCL patients without COVID-19 were vaccinated against SARS-CoV-2 during the period from January to October 2021. By the time of HCL diagnosis, the patients were aged 23–74 years (median 47 years), whereas by the time of COVID-19 diagnosis, the age range was 25–82 years (median 55 years). There were 63 men and 36 women.
 Results. Severe CT 3–4 lung lesions were diagnosed in 22 (22 %) patients and detected more often (55 %) prior to therapy for advanced-stage HCL. In patients on therapy for advanced-stage HCL and those in remission, the rate of severe lung lesions (CT 3–4) was 17 % in each group. COVID-19 therapy was administered to 49 (49 %) patients in a special hospital. Mortality in the total cohort was 6 % (n = 6) and differed in patients with advanced-stage HCL (n = 4) and those in remission (n = 2). The patients who received anti-CD20 monoclonal antibodies for < 12 months prior to COVID-19 infection, in 42 % (n = 5) of cases showed seronegativity vs. 5 % (n = 1) of patients who received the same therapy > 12 months before COVID-19 was diagnosed.
 Conclusion. The most severe COVID-19 was observed in chemotherapy-free patients with advanced-stage HCL compared to patients on therapy or in HCL remission. When anti-CD20 monoclonal antibodies were administered for < 12 months prior to COVID-19 diagnosis, the number of patients with negative anti-SARS-CoV-2-IgG immune response increased 15-fold. Mortality in novel coronavirus infection COVID-19 differed at various HCL stages (in remission and at advanced stage).