Abstract
We performed an ecological study using sentinel consultation data from a medical deputising service to assess the impact of increasing coverage with childhood varicella vaccine on the incidence risk of varicella and zoster in the population served by the deputising service in Victoria, Australia from 1998 to 2012. Following a successful vaccination programme, the incidence of varicella in Australia was modelled to decrease and the incidence of zoster to increase, based on a theoretical decrease in boosting of zoster immunity following a decrease in wild varicella virus circulation due to vaccination. Incidence risks (consultation proportions for varicella and zoster) were directly age-standardised to the Melbourne population in 2000, when varicella vaccine was first available. Age-standardised varicella incidence risk peaked in 2000 and halved by 2012. Age-standardised zoster incidence risk remained constant from 1998 to 2002, but had almost doubled by 2012. The increase in zoster consultations largely reflected increases in people younger than 50 years-old. Although causality cannot be inferred from ecological studies, it is generally agreed that the decrease in varicella incidence is due to increasing varicella vaccine coverage. The possible indirect effect of the vaccine on zoster incidence is less clear and ongoing monitoring of zoster is required.
Highlights
In 1998 the World Health Organization recommended adding varicella vaccine to routine childhood vaccination schedules where it could be shown to be of relative public health and socio-economic importance, where it was affordable and where sustained coverage could be achieved [1]
This possibility is based on the hypothesis that T-cell-mediated immunity to zoster is boosted by repeated exposure to circulating wild varicella virus and that this boosting will decrease following the decrease in circulating wild virus due to varicella vaccination
This is known as the Hope-Simpson hypothesis [7]. Assuming this hypothesis to be true, modelling in England [8,9,10], Finland [11], the United States [12] and Australia [13] suggested an increase in zoster incidence for as many as 60 years would accompany a decrease in varicella incidence following widespread use of varicella vaccine in childhood
Summary
In 1998 the World Health Organization recommended adding varicella vaccine to routine childhood vaccination schedules where it could be shown to be of relative public health and socio-economic importance, where it was affordable and where sustained coverage could be achieved [1]. Part of the reluctance to introduce universal varicella vaccination in some European countries was related to the theoretical possibility that high coverage with varicella vaccine in children would lead to an increase in zoster. This possibility is based on the hypothesis that T-cell-mediated immunity to zoster is boosted by repeated exposure to circulating wild varicella virus and that this boosting will decrease following the decrease in circulating wild virus due to varicella vaccination. Zoster vaccine was not available in Australia during the years of the study (1998 to 2012)
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