2.3. Predisposing factors

Individuals with potential predisposing factors were identified from the registers by one or more of the following definitions: 1) having an entitlement to reimbursement of medicine expenses due to a specified comorbidity, 2) using specific medication or 3) using health care services due to the specified comorbidity. We used a pre-defined list of diagnoses and codes referring to specific comorbidities, medical conditions or the use of pharmacotherapy (Table 1 ). The following registers were employed: the Care Register for Health Care, the Register of Primary Health Care Visits, as well as the Special Reimbursement Register for Medicine Expenses and the Prescription Centre database, both maintained by the Social Insurance Institution of Finland. The registers used in the identification of each potentially predisposing factor are listed in the Table 1 at column ‘Register’. The dataset covers for the most part all health care providers in Finland. The dataset was linked to the Finnish Population Information System in order to identify those without any of the factors potentially predisposing to severe COVID-19 listed in Table 1.

Comorbidities and pharmacotherapies considered as potentially predisposing factors to severe COVID-19.

ATC, Anatomical Therapeutic Chemical Classification System; ICD-10, International Statistical Classification of Diseases and Related Health Problems, tenth revision; ICPC-2, International Classification of Primary Care, second edition; NCSP, Nordic Nomesco Classification of Surgical Procedures.

Registers: 1, Special Reimbursement Register for Medicine Expenses; 2, Care Register for Health Care; 3, Register of Primary Health Care Visits; 4, Prescription Centre database.

Individuals entitled to the specified special reimbursement of medicine expenses were identified from 1 January 2020 onwards. In Finland, all residents entitled to special reimbursement must have a condition that meets a set of specified medical criteria ensured by a medical certificate from a physician.[17] Individuals using medication predisposing to immunodeficiency or using medication for type 1 or 2 diabetes not more than two years prior to testing positive for SARS-CoV-2 were identified based on the Anatomical Therapeutic Chemical (ATC) codes.

We identified individuals who had used health care services due to any of the specified comorbidities not more than two years (cancer) or five years (all other comorbidities) prior to the positive PCR test. Secondary health care services were identified with the International Classification of Diseases 10th revision (ICD-10) codes and the Nordic Classification of Surgical Procedures (NCSP) codes. Primary health care services were identified with ICD-10 codes and the International Classification of Primary Care Second edition (ICPC-2) codes.