This is likely an over-estimation of the proportion of episodes that are recurrent. A study that validated diagnoses and which included a 12 year follow-up, found that recurrence occurs in about 6% of cases [16]. Some of the episodes that we classified as recurrent may have been misclassified despite our requirement of a minimum of 180 days between visits in our case definition of recurrence. Misclassification could also

have occurred due to selleck chemicals llc coding errors for a different true diagnosis or because a herpes zoster code was used for a situation in which the clinician had indicated only a past history of disease. This has been observed elsewhere [16]. We were not able to validate the shingles diagnostic codes used in this study. A comparison of administrative data to medical records in the United States found that using administrative data alone resulted in a zoster occurrence rate that was inflated by 17.4% (95% CI 15.4, 19.5) and an absolute difference in incidence of 0.78/1000 person years [16]. However, we used similar methods to ascertain cases in both the pre- and post-vaccine eras and do not anticipate that it would affect the patterns observed. We acknowledge that we may have over-estimated shingles rates among children as it has

also been shown that the validity of a shingles diagnosis from administrative selleck screening library data varies by age and is lower among younger than older persons; particularly for younger children [17]. We perceive that one of the impacts of effective chickenpox vaccination programs will be that clinicians may become more likely to misdiagnose both chickenpox and shingles over time in younger persons; the implementation of shingles vaccination programs

heptaminol may have a similar impact among older persons. Thus it is increasingly important that validation studies of administrative data be done on an ongoing basis and further, as diseases become less common the use of more highly specific case definitions will be important. Our study did not capture cases of shingles that did not seek medical care; we are not able to estimate this proportion but it is possible that this proportion might have decreased over time if public awareness of treatments for shingles has changed over time. The risk factors responsible for the overall trend of increasing shingles rates that began prior to chickenpox vaccination are not understood, although changes in age and immune status of populations are thought to be inadequate to explain them [18]. Ongoing surveillance of both chickenpox and shingles are essential, but other factors make epidemiologic interpretation increasingly complex, including dosing schedules for chickenpox and shingles vaccines, population mixing patterns by age group and sex, and possible changes in the virus itself. Alberta introduced a second dose of chickenpox vaccine into the routine childhood vaccination schedule in August 2012 [7].