Dear Editor,

By October 2021, high mortality rates had been reported in patients with hematologic malignancies (HM) who contracted COVID-19 [1,2,3]. The B.1.1.529 variant, named Omicron, emerged in November 2021 and replaced Delta as the dominant variant worldwide. The Omicron variant has been reported to cause lower rates of hospitalization, severe disease, and mortality compared with other variants [4, 5], but the prognosis in patients with HM is unclear. Our study reports our experience of a nosocomial outbreak in nine patients with HM who were infected with the Omicron variant at our hospital.

An outbreak of COVID-19 occurred in our hematology ward in February 2022 following the admission of a pre-symptomatic patient. A total of nine patients were diagnosed with COVID-19 by SARS-CoV-2 reverse quantitative polymerase chain reaction testing of nasopharyngeal swabs. The nasopharyngeal swabs of eight of the nine patients were tested for S-gene targeting failure (SGTF); these eight patients had SGTF. All nine cases were diagnosed as Omicron variant infection because the prevalence of Omicron was greater than 95% in Kitakyushu City at that time, and all infections had occurred within a few days of each other.

The mean (± standard deviation) age of the nine patients was 74 ± 7 years (range, 61 to 85 years), and six (66.7%) were male. All subjects had active HM: 3 (33.3%) with myeloproliferative disease, 4 (44.4%) with malignant lymphoma, and 2 (22.2%) with multiple myeloma. Six (66.7%) patients had received two doses of vaccine more than 6 months ago. All survivors were followed up for at least 17 days. According to the National Institutes of Health severity classification [6], one (11.1%) patient was asymptomatic, and illness was mild in three (33.3%), moderate in two (22.2%), and critical in three (33.3%). Each attending physician decided the treatment options among sotrovimab, remdesivir, dexamethasone, high-dose corticosteroids, baricitinib, and tocilizumab, taking into account the severity of COVID-19, organ damage, and comorbidities. Two (22.2%) deaths occurred, and these were attributed to COVID-19. Two of the survivors remain critically ill and their situation is life-threatening (Table 1).

Table 1 Patient characteristics
Full size table

A study that analyzed data from patients with HM who were infected with COVID-19 in the year after March 2020 (before the appearance of the Omicron variant), 63.8% of cases had a severe or critical illness and 22.2% died due to COVID-19 [3]. Compared with that study, the present report showed a similar mortality rate despite the lower severity of the disease. Furthermore, both of the critical patients were in a life-threatening situation at the time that this report was submitted, meaning that the mortality rate could possibly increase further. Side effects of treatment for HM and comorbidities may increase mortality in hospitalized patients with HM. In fact, a patient with dysphagia died from aspiration of sputum despite having only moderate severity of the disease.

In conclusion, although the Omicron variant may be less severe than previous variants, the mortality rate of hospitalized HM patients infected with the Omicron variant remains high. It is important that infection control measures are not neglected despite the development of vaccines and the implementation of treatment measures.