A decline in varicella but an uncertain impact on zoster following varicella vaccination in Victoria, Australia
Introduction
Varicella vaccination is increasingly being incorporated into childhood immunisation schedules around the world. Modelling studies have predicted that, following the introduction of varicella vaccine delivered as a population-based program, there would be an immediate decrease in the number, but an increase in the median age, of childhood cases of varicella [1], [2], [3], [4]. This would occur even for low levels of vaccine coverage. The models also assumed that repeated exposure to varicella zoster virus (VZV) protected adults from herpes zoster, and consequently predicted an increase in zoster cases in the years after introduction of the varicella vaccine due to decreased circulating virus in the community and reduced virus exposure for older people.
Varicella vaccination was introduced in the United States in 1995 and, while there have since been substantial reductions in varicella incidence in accord with the models [5], [6], [7], there have been mixed reports about the impact of the program on zoster. Two reports from different areas using hospital administrative data found no change in zoster incidence rates up to 2002 [8] and to 2003 [9], with increases in zoster among children in the latter study attributed to the use of oral steroids. However, increased zoster rates up to 2003 were found with a telephone survey [10] and up to 2004 in an analysis of data from the Nationwide Inpatient Sample [11]. Complicating our understanding of changes in zoster incidence are the studies from Canada and the UK describing recent increases in zoster independent of the implementation of varicella vaccination [12], [13].
Varicella vaccine was licensed in Australia for children from 12 months of age in 1999 (available 2000) and was scheduled and funded on the National Immunisation Program (NIP) in November 2005. One dose of vaccine at 18 months (12 months once the measles, mumps, rubella and varicella vaccine is in use) and a catch-up dose at 10–13 years of age were recommended [14]. In 1998, the year prior to the introduction of varicella vaccine into Australia, VZV infection resulted in approximately 2000 hospital admissions for varicella and 8 deaths among hospitalised patients. VZV was also responsible for approximately a further 4500 hospital admissions for zoster and 45 deaths among hospitalised patients [15]. In Australia the increase in zoster cases was modelled to start within a year of the commencement of a varicella vaccination program and to last for 59 years, before declining [16]. We aimed to examine trends in age-specific incidence of hospitalisation of both varicella and zoster in Victoria, a state comprising around 25% of the Australian population [17], from 1995 to 2007. To assist in the interpretation of our findings, we compared hospitalisation rates for varicella and zoster with community sourced data on the number of weekly calls diagnosed as chickenpox or shingles by locum doctors working with the Melbourne Medical Deputising Service (MMDS).
Section snippets
Methods
Data associated with any diagnosis of varicella or zoster were extracted by staff of the Victorian Department of Health from the Victorian Admitted Episodes Dataset (VAED) for the calendar years 1995–2007 inclusive. The VAED collects morbidity data on all admitted patients from Victorian public and private acute hospitals. The International Statistical Classification of Diseases and Related Health Problems, Tenth Revision, Australian Modification (ICD-10-AM) was applied to episodes separated
Varicella
There were 3853 admissions to Victorian hospitals for varicella from 1995 to 2007, 2483 (64%) with varicella as the principal diagnosis. Varicella hospitalisations increased in Victoria from 1995 to 2000 and subsequently declined (Fig. 1). The same trends were seen in each phase of vaccine availability whether all diagnoses or only principal diagnoses (the main reason for admission) were considered (Table 1). Over the complete period in which vaccine was available (privately or publicly,
Discussion
We have shown that since varicella vaccine has been available in Victoria there has been a significant decline in the varicella hospitalisation rate, due mainly to a decline in children aged under five, the age group targeted for vaccine. There may be some suggestion of an accelerated decline in hospitalisations of young children and 20–49 year olds in the two years following public funding of the vaccine. Analysis after a longer period of public funding will be necessary to confirm these short
Acknowledgements
We thank CSL and GSK staff for vaccine distribution figures, the Victorian Department of Health for hospitalisation data and vaccine distribution data, the Melbourne Medical Deputising Service for locum consultation data, and Michael Coory for statistical advice.
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2016, Journal of InfectionCitation Excerpt :This is in agreement with two previous studies about the incidence of hospital admissions for England.23,24 However, the majority of studies that compared the burden of hospital admissions due to varicella and zoster found that the incidence of admissions was higher for zoster than for varicella,25–29 independent of the method used for case ascertainment. This could be caused by differences in the severity of varicella-zoster infections due to the ethnic composition of the English population or differences in the health care systems.