Epidemiology of herpes zoster

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Population-based epidemiologic data suggest an annual incidence of HZ between 1.3 and 4.1 per 1000 population [18-20]. Some longitudinal studies suggest an increase in incidence in the last two decades although the reason for this is not clear [5, 20]. The incidence is much higher in the elderly; a recent study suggests an incidence rate of 2.1 per 1000 person-years in those under 50 and 10.1 in those over 80, a fivefold difference [20]. Other large studies show a similar trend [1, 19]. These figures are generally accepted to reflect the natural decline in cell-mediated immunity with advancing age. All reported studies rely on the clinical presentation of HZ. Some studies suggest that 10% of the diagnoses of HZ made in primary care are in fact due to zosteriform herpes simplex. Other common misdiagnoses include common dermatologic diseases such as contact dermatitis, erysipelas and insect bites [5, 21]. By contrast, atypical forms of herpes zoster also exist, such as zoster sine herpete, in which rash does not develop.

Immunocompromised patients represent 8-10% of HZ cases [20]. Some investigators have proposed that childhood varicella vaccination will alter the incidence figures considerably when it is more widely adopted. It is thought that the significant reduction in the incidence of varicella resulting from vaccination reduces the chance of exposure of the elderly to the exogenous viral antigen pool needed to boost their cell-mediated immunity. This fact combined with the rapid increase in the number of the elderly and immunocompromised patients is likely to lead to a short-term increase in herpes zoster cases [5]. However, if adult HZ vaccination becomes widely practiced as is expected, the long-term outcome is likely to be a significant reduction in HZ incidence.

Postherpetic neuralgia is the most common complication of herpes zoster. Estimates of its incidence come from both prospective community-based or large retrospective population-based studies. Hall and others evaluated data from the computerized UK general practice records and reported an annual incidence of 40 per 100,000 person-years [2]. This is higher than the incidence calculated from GP records in London of 11 per 100,000 person years [22] but similar to another British population study [23]. In line with incidence of HZ, PHN at 1 month was reported far more frequently by people aged 65-74 (11%) or over 75 (18%) than those 45-54 (4%) [1]. PHN usually resolves spontaneously, leaving a small percentage of patients to suffer from chronic pain.

Before the era of antiviral treatments, reports of the presence of PHN pain sufficient to induce a visit to the physician were 5-7% at 3 months, 3-5% at 6 months and 2-4% at 12 months [18, 24, 25]. Newer data obtained while antivirals were widely available suggest even lower figures. Helgason and co-authors collected data over a 6-year period on 421 patients representing a rural region in Iceland with approximately 100,000 inhabitants [26]. Only 2% of those under 60 years of age reported pain at 3 months which in all cases was mild. Thirty percent of patients over 70 reported pain at 3 months, 11% moderate or severe. At 12 months one patient (1%) in the 60-69 age group and one (2%) patient in the over-70 group reported moderate pain, and none had severe pain. At 12 months 3.3% reported pain, mostly mild. Two patients (0.5%) reported moderate pain. Of note, only 4% received antivirals [26]. Similar results were reported from two opportunistic patient populations. Haanpaa et al. reported that at 6 months five of their original sample of 113 patients of all ages with HZ had pain of moderate or severe intensity [27]. Similarly, Thyregod and co-workers found at 6 months following the original rash that only two of the original 94 patients had clinically meaningful pain, which they defined at >3/10 [4]. These examples do not reflect the reality seen in many pain clinics who manage patients with intractable PHN. The incidence in the era of antivirals of this extreme form of PHN remains unknown but in all likelihood is very low. Nevertheless, as HZ remains a very common condition (with a 30% lifetime risk in developed countries [28]), there are likely to be a sufficient number of patients with disabling PHN who need specialized help and active pain management.

The prevalence of postherpetic neuralgia in the general population is unknown. One retrospective study based on GP records of patients of all ages in London estimated the lifetime prevalence for PHN (defined as pain at 1 month after the rash) to be 0.7 (95% confidence interval (CI) 0.4-1.0) per 1000 population [22]. Bowsher based his estimate on the personal histories obtained from a community survey of a cohort of 1071 people aged 64-99, and arrived at a figure of 25/1000 in that age group [29]. No estimate of severity of pain was attempted. An example of the difference of pain reported in surveys and those leading patients to seek help from their doctors comes from another study. In a prospective study of 598 patients with acute HZ and aged over 50, 16% had pain of any severity at 6 months, and 10% still reported it at 4 years [30]. However, severe pain was reported by 2% at 6 months and 0.7% (1 out of 139) at 4 years, underlining the vast difference between clinically relevant and other pain.

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