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Introduction
Herpes zoster (HZ) is a viral disease caused by reactivation of varicella-zoster virus (VZV) that remains latent in cranial-nerve or dorsal-root ganglia after previous varicella infection. HZ is characterized by a painful, dermatomal, vesicular rash that in most cases resolves without sequelae within about 1 month [1, 2]. Importantly, approximately 25% of all patients experience HZ-associated complications, the most common of which is postherpetic neuralgia (PHN), persistent pain in the affected area that may last for months or years and severely reduce the patient’s quality of life and ability to perform their usual activities of daily living [3, 4]. Other common HZ complications are ocular, neurological (other than PHN), and disseminated HZ [5, 6].
The cause of reactivation of VZV has not been clearly elucidated, but it is known that effective VZV-specific cell-mediated immunity is necessary to keep VZV in a dormant state [5]. Reactivation may occur at any age, but increasing age is the most important risk factor for HZ, as a result of natural immunosenescence that advances markedly from the age of about 50 years. The proportion of HZ patients developing PHN and other complications also increases with age [5, 7]. Other immunocompromising (IC) conditions, such as malignancies and immunosuppressive medical treatments or interventions often used to treat autoimmune diseases and in organ transplant recipients, may also lead to increased risk of HZ [8, 9, 10, 11–12].
The epidemiology of HZ and its complications is relatively similar around the world (although estimates of the frequency of PHN vary widely due to varying definitions [13]), but the economic burden of the disease varies considerably between countries, depending on factors such as the level of socioeconomic development and types of healthcare available [14].
In Japan, studies have shown an incidence of HZ in the general population aged ≥ 50 [15] or ≥ 60 years [16] of approximately 10 per 1000 person-years (PY), increasing to more than 12/1000 PY in people aged ≥ 80 years. The overall proportion of HZ patients developing PHN was found to be 19.7% in one study, increasing to 32.9% in individuals aged ≥ 80 years [15]. Sato and colleagues found that the odds ratio of PHN development in people on current immunosuppressive treatment compared with no such treatment was 6.44...