The database searches identified 1045 records (776 unique records after removing duplicates) for screening with an additional 79 references from the reference lists of systematic reviews identified. A total of 652 abstracts and 137 full-text reports (65 from database searches and 72 from additional references) were not eligible and were excluded, and it was not possible to retrieve seven full-text publications. Reasons for exclusion of full-text reports from the searches are described in Supplemental file 3. Overall, 59 studies were selected for inclusion in the systematic review (Fig. 1), from ten countries in Europe (Austria, France, Germany, Greece, Italy, the Netherlands, Spain, Sweden, Switzerland, and the United Kingdom). Baseline characteristics of the included studies can be found in Table S1 (Supplemental file 4). There were 26 studies reporting the primary outcomes of interest i.e., 24 reported HZ incidence rate [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48], and two studies reported HZ cumulative incidence [49, 50] (Table 1). The remaining 33 studies reported secondary outcomes i.e., HZ relative incidence [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83]. These mostly focused on the occurrence of different types of infection, including HZ, after a specific treatment or a transplant. Therefore, these secondary outcomes were not the main focus of this review and are reported in Table S2 (Supplemental file 4).

PRISMA flowchart. *Full-text article checked if eligibility not clear from abstract. **Includes studies combining relevant and non-relevant countries/IC populations without stratified data. HZ herpes zoster, IC immunocompromised, PRISMA Preferred Reporting Items for Systematic Reviews and Meta-analyses, SLR systematic literature review, VZV varicella zoster virus

Within the categories of high-risk populations presented in Table 1, there was variability in the reported HZ incidence across studies, partly due to different subgroups of patients or different immunosuppressive medication. The study findings are described in more detail by population and subgroup below, starting with the populations with the highest reported HZ incidence rates (per 1000 person-years). Subgroup comparisons within these populations were not statistically compared unless otherwise indicated. Some studies provided comparisons between IC and non-IC populations, and while the focus of this research was only on IC populations, comparative results are presented when available.

Seven studies [30, 34, 37, 38, 41, 47, 48] reported HZ incidence rates in the subgroup of solid organ transplant recipients, all studies focused on adult patients (≥ 18 years). The HZ incidence rate per 1000 person-years ranged from 12.1 in a mixed group of solid organ transplant patients [48] to 78.8 in kidney transplant patients [30].

A retrospective study (N = 1033) reported incidence rates by type of solid organ transplantation [34]. The highest incidence was found in patients with a lung transplant (38.8), followed by heart transplant (30.7), liver transplant (22.7), and kidney transplantation patients (14.5) [34]. Another retrospective study in lung transplant patients (N = 119) reported similar incidence rates (38.2 [47] vs. 38.8 [34]). A prospective study (N = 444) [30] and a retrospective study (N = 450) [38] in kidney transplant patients reported higher incidence rates (20.6 [38] and 78.8 [95% CI 54–104] [30]). All three studies in kidney transplant patients [30, 34, 38] included adult outpatients with HZ diagnosed based on clinical signs. In one of these studies, incidence was found to be higher in patients ≥ 60 years (43) versus < 60 years of age (16.7) [38]. Three studies [37, 41, 48] reported incidence rates in a group of combined solid organ transplant patients (i.e., ranging from 12.1 [48] to 14.5 [41] in transplant patients versus, for non-IC patients, 4.6 [95% CI 4.6–4.7] in Spain [37], 5.9 [95% CI 5.8–5.9] in Germany [41], and 6.2 [95% CI 6.1–6.3] in England [48]). Higher incidence rates were found in the older age categories (e.g., 17.0 in 60–69 years and 17.3 in ≥ 80 years versus 6.0 in 18–29 years of age in Spain [37], and 16.9 in ≥ 80 years versus 11.0 in 18–49 years of age in England [48]).

Three studies [37, 41, 48] reported HZ incidence rates in stem cell transplantation patients aged ≥ 18 years, ranging from 37.2 (95% CI 32.0–43.0) [41] in in- and outpatients in Germany receiving stem cell transplantation (including HSCT) to 56.1 (95% CI 48.9–64.0) [37] in HSCT patients in Spain. Incidence rates were significantly lower in non-IC patients (e.g., 5.9 [95% CI 5.8–5.9] in Germany [41], 4.6 [95% CI 4.6–4.7] in Spain [37], and 6.2 [95% CI 6.1–6.3] in England [48].)

Incidence rates reported peaked in 50–59 year-olds (69.2 vs. 5.5 in non-IC patients) in Spain [37] and 60–64 year-olds (60.3 vs. 6.9 in non-IC patients) in England [48]. Lower rates were reported in younger age groups (42.4 in 18–29 year-olds [37] and 34.1 in 18–49 year-olds [48], vs. 2.1–2.3 in non-IC patients [37, 48]), as well as in older age groups (52.0 in 70–79 year-olds [37] and 47.0 in 65–69 year-olds [48], vs. 8.6–9.3 in non-IC patients [37, 48]).

Four studies [