*2.3. Exposure Measurement*

Influenza vaccination was defined using an ICD-9-CM (V04.7 or V04.8) diagnosis or a drug code within two years before the index date. Comorbidities considered were hypertension (ICD-9-CM 401–405), diabetes mellitus (ICD-9-CM 250), cerebrovascular disease (ICD-9-CM 430–438), renal disease (ICD-9-CM 582–582.9, 583–583.7, 585, 586, and 588–588.9), liver disease (ICD-9-CM 456.0–456.21, 571.2, 571.4–571.49, 571.5, 571.6, and 572.2–572.8), ischemic heart disease (ICD-9-CM 410–414), dementia (ICD-9-CM 290.0–290.4, 294.1, 331.0–331.2, and 331.82), alcohol-related disorder (ICD-9-CM 291, 303, 305.0, 571.0, 571.1, 571.3, 790.3, and V11.3), and malignancy (ICD-9-CM 140–208). All patients were diagnosed with the disease before the index date.
