Background: Follow-up of patients who had coronavirus disease 2019 (COVID-19) proves that clinical symptoms persist for months after recovery. A complex of such persistent manifestations is defined as the post-COVID-19 syndrome. One of the criteria for post-COVID-19 syndrome may be typical changes in white blood cell count and white blood cell (WBC) differential. The aim of the work is to study the frequency of haematological changes in sailors who had the acute coronavirus infection. Materials and methods: The retrospective study covered 30 candidate sailors aged 21 to 60 years with a history of COVID-19 and persistent changes in the WBC count and WBC differential and who did not have haematological abnormalities during the previous medical examinations. Results: Analysis of WBC and WBC count at the long-term period after COVID-19 confirmed persistent changes in the form of neutrophilia, lymphopenia, changes in the neutrophils and lymphocytes ratio. The revealed changes in the WBC count were typical and fit into several patterns: A. Absolute leukocytosis, absolute and relative neutrophilia, relative lymphopenia; B. Relative and absolute lymphopenia, relative neutrophilia; C. Relative and absolute lymphocytosis, relative neutropenia; D. Relative lymphopenia, without other changes in WBC differential. Conclusions: The most typical laboratory change in WBC count in patients with the past COVID-19 is relative or absolute leukopenia. Persistent changes in WBC count are not always outside of the reference range for absolute values and should be assessed by a complex of typical changes. The presence of typical changes in WBC count in a patient with the past COVID-19 requires a profound examination for the post-COVID-19 syndrome.