Study Results in Context of Previous Findings
Using German claims data, we have conducted an analysis of patients with HZ and assessed their risk for a recurrence in relation to UCs. The results indicate that patients with either asthma, CHD, COPD, depression, or RA had an increased risk of developing an acute HZ compared to patients without any of the UCs under study. RA had the highest OR among these conditions, varying from 1.37 to 1.57 for all age groups, and patients with depression also showed a significantly higher risk of developing HZ in all age groups. When the data were adjusted for systemic corticosteroid therapy in patients with asthma and COPD, the risk for an acute HZ was higher in those using systemic corticosteroid than in those who were not. Analysis of recurrence indicated that patients with at least one UC in the age groups 18–49 years and 50–59 years had the highest risk for a recurrent HZ. Regardless of age groups, the risk of developing a second recurrence was two- to threefold higher as compared to the risk of developing a first recurrence.
Multiple studies have related UCs to an increased risk of HZ [
13,
16,
17,
33‐
47]. In addition, a recent meta-analysis showed robust associations between HZ and UCs across 88 studies [
48]. The present study corroborates these results. The related risk for HZ can be explained by the diseases themselves and their underlying biological mechanisms, as well as by specific immunosuppressive drug treatment.
Our results on the effect of RA on HZ are consistent with those reported from other studies. A case–control study from the UK using primary clinical practice data reported an OR very similar to the results of the present study (OR 1.46) [
13]. In RA, it is not only the general impairment of the immune system that is associated with an increased risk for infectious diseases, but the disease activity of RA itself plays an important role in the risk of contracting an infectious disease [
49]. There is a high prevalence of multi-morbidity, such as depression, asthma, and/or COPD, in patients with RA, which can further impair the immune system [
50]. Immunosuppressive medications, such as corticosteroids, azathioprine, cyclophosphamide, or tumor necrosis factor inhibitors, which are used for the treatment of RA, are also associated with an increased risk of infections [
35,
49]. This latter association has been investigated for HZ in an earlier study, which reported that the highest risk for HZ infections was associated with former cyclophosphamide use (OR 3.17) [
51].
Numerous international studies have investigated whether diabetes mellitus is a risk factor for HZ. In accordance with the results presented here, several studies have shown that younger patients with diabetes appear to have an increased risk of HZ when compared to older patients with diabetes [
13,
39,
41,
43]. For example, Forbes et al. found that those with the strongest association between HZ and DMT1 (OR 1.51) and HZ and DMT2 (OR 1.22) were those aged less than 50 years [
13]. A number of studies have examined the association between HZ and diabetes and shown contradictory results [
16,
18,
41]. The varying risk for HZ across studies can be due to different methodological aspects (i.e., the underlying definition of age groups for patients younger than 50 years, or adjustments for age and comorbidities); for example, one study showed that diabetes patients with high glycated hemoglobin (HbA1c) and aged less than 45 years have an increased risk for HZ [
43]. However, laboratory data are usually only available in primary data studies.
The observation of increased risk for HZ in patients with depression is consistent with results reported in international literature, as reported in a recent meta-analysis (relative risk 1.23) [
48]. A retrospective case–control study based on UK primary care data showed that patients with depression younger than 50 years are at a higher risk for developing HZ (OR 1.24) [
13]. Two other studies also observed an increased risk in patients with depression, but did not include different age groups in their analysis [
34,
45]. Although there seems to be an association between depression and risk of HZ, the explanation still remains unclear. It is possible that there is a decreased and dysregulated cell-mediated immunity in patients with depression, as it has been shown that depression is associated with reduced lymphocyte proliferation and decreased proportions of lymphocytes and T-cells [
44]. Irwin et al. support this association by showing that VZV-specific responder cell frequency, a marker for cell-mediated immunity, is especially reduced in older patients with major depressive disorders [
52,
53]. The observed association for younger age groups and their cell-mediated immunity as well as their specific risk for HZ requires further research.
The data show that COPD and asthma patients had a significantly higher risk of HZ when using systemic corticosteroids. The increased risk for HZ in these patients may be explained by the immunosuppressive effect of corticosteroids, resulting in COPD and asthma patients being susceptible to infectious diseases. Systemic corticosteroids are associated with negative effects on immunity, including the inhibition of inflammatory cytokines, reduction of antigen-presenting cells, and decrease of T-cells [
33], consequently increasing the susceptibility to infections.
Recurrence rates for HZ have been previously reported by other authors, yet differences in study designs, study populations, and length of follow-up makes it difficult to compare results [
18,
19,
54‐
57]. Furthermore, there are no other studies reporting the risk for a second recurrence or recurrence for patients with UCs. However, a recent study from Spain reported significantly higher recurrence rates in immunocompromised patients [
58]. This result is in line with the results presented here which show that patients with UCs have a higher risk of recurrence. Kim et al. reported overall recurrence rates of 12.0 per 1000 PY [
18], and Tseng et al. reported a 10-year cumulative recurrence rate of 10.26% [
54]. Similar to the results in this study, these authors observed a higher probability of recurrence in younger age groups compared to older adults. In the present study, this effect is even more pronounced in patients with at least one UC.
Limitations
The study presented here has a number of limitations. It must be considered that the underlying data are collected for billing purposes. In the literature, the potential for upcoding is discussed for some of the considered UCs [
59] and, therefore, it is possible that the risk of HZ in patients with UC is overestimated. An increased likelihood of receiving a diagnosis of HZ, for example, might result from regular visits to a general practitioner for a chronic condition. It is reasonable to assume that most patients with HZ would make an appointment with their general practitioner owing to the extensive rash and considerable pain associated with HZ, and also because everyone living in Germany has healthcare insurance. Furthermore, laboratory confirmation is missing and, therefore, other diagnoses cannot be ruled out, mainly zosteriform herpes simplex, which is especially relevant for recurrent HZ as it is associated with milder clinical symptoms than primary HZ [
19]. Also, a number of risk-modifying factors for HZ incidence, such as the Hb1Ac value or other morbidity-associated laboratory parameters, cannot be captured in the claims database. Another limitation of the study is that it was not possible to adjust the data for some factors that may have affected the prevalence of considered UCs, including smoking, alcohol consumption, physical activity, and dietary habits. In addition to clinical aspects of risk-modifying factors, UC may also adversely impact the QoL in patients suffering from HZ. However, effects on QoL could not be assessed in the present study based on the available data. As the database is limited to one SHI and individuals in Germany self-select their SHI, it is important to assess the representativeness of the database. A comparison with national population statistics shows that those insured by BARMER are representative in terms of age and gender, as confirmed by national health technology assessment institutions [
60].