Since December 2019 more than 460 million confirmed cases and 6 million deaths have been attributed world-wide to coronavirus disease 2019 (Covid-19)[1]. Patients with hematological cancer have been reported to suffer increased risk of severe Covid-19, with a hospitalization rate of more than 50% and a case fatality rate of approximately 30%[2]. The impact of Covid-19 on patients with chronic myeloid leukemia (CML) remains uncertain [2–5]. The Italian Campus CML network reported on more than 8000 CML patients and detected only 217 Covid-19 positive cases [5]. However, among these cases 22% were admitted to hospital with a relatively high mortality rate of 5.5%. Similarly, a preliminary report from the CANDID study, conducted by the International CML Foundation, estimated the mortality rate among Covid-19 infected CML cases at 14%[4]. Because of these somewhat diverging data, a recent review concluded that the incidence and severity of Covid-19 in CML have not yet been thoroughly investigated and called for a true population-based study [6]. Our goal was to perform such a study, examining a large number of patients and comparing against carefully matched controls, utilizing high-quality full-coverage population-based registers addressing the period when Covid-19 was classified as “disease dangerous to public health” with widespread public PCR testing was available, February 1st 2020 to April 1st 2022. We utilized an existing database containing data on all Swedish individuals together with information on relevant Covid-19 outcomes (the SCIFI-PEARL project database), with information on all positive Covid-19 tests (PCR-based testing) as well as all Covid-19-related hospital and intensive care unit (ICU) admissions, deaths, and national vaccination data [7]. This database specifically contains exposure and outcome information in terms of information on specialized outpatient-and all inpatient healthcare, cancer diagnoses, as well as information on all dispensed drug prescriptions [8].

We defined two separate CML cohorts. The first, main cohort, consisted of individuals diagnosed with TKI-treated CML in chronic phase (CP-CML) before March 1st 2020, defined here as the start of the pandemic in Sweden. A secondary cohort consisted of individuals diagnosed during the pandemic, after this index date. We excluded individuals born outside of Europe, as it is documented that they suffered increased susceptibility to severe Covid-19 outcomes in the Swedish population [9]. Using the same database, we selected five controls for each CML patient in the two cohorts by randomly matching for age (within 60 days), sex, and county of residence on March 1st 2020. The individuals in the two CML and control cohorts were followed from March 1st 2020 to the first event of each outcome studied, April 1st 2022, emigration or death, whichever occurred first. This effectively means that follow-up for patients diagnosed during the pandemic was also initiated at the start of the pandemic and not at the time of the CML diagnosis–allowing to capture potentially more severe Covid-19 cases even if diagnosed before the CML disease–a disease that is diagnosed in the asymptomatic case in half of the patients and that has existed and affected the body for years before the diagnosis date.