Impact of single-dose universal varicella vaccination program on paediatric varicella-zoster virus-related hospitalisations in Australia, 2002–2022

BACKGROUND: Australia introduced a single-dose varicella vaccine at 18 months of age on the National Immunisation Program in 2005. With this study, we aimed to assess the long-term impact of this program on paediatric varicella and zoster hospitalisations and complications, and to describe trends in immunocompromising conditions among hospitalised cases. METHODS: We obtained hospitalisations among children aged <15 years with a varicella (B01) or zoster (B02) ICD-10-AM code during 2002-2022 from the Australian Institute of Health and Welfare National Hospital Morbidity Database. Following a literature review, we identified hospitalisations with varicella- or zoster-related complication code(s), and hospitalisations with concurrent code(s) indicating immunocompromise. We calculated hospitalisation rates, rate ratios, and the proportion of hospitalisations with complication or concurrent code for immunocompromise, by age group. FINDINGS: Immediately following introduction of the varicella vaccination program, paediatric varicella hospitalisation rates rapidly declined, with a subsequent more gradual decline. In the 2015-2019 period, varicella hospitalisation rates were 81%, 94%, 88% and 70% lower than the 2002-2005 rates in the age groups <18 months, 18-59 months, 5-9 years and 10-14 years, respectively. Zoster hospitalisation rates declined over the study period in the age groups 18 months-14 years of age. There was a reduction in the rates of varicella and zoster hospitalisations with complication. The proportion of varicella hospitalisations with coded immunocompromising condition increased, though the absolute number of such hospitalisations decreased, whereas this proportion (and absolute number) decreased for zoster hospitalisations. DISCUSSION: A substantial reduction in the burden of paediatric varicella and zoster was observed following introduction of Australia's single-dose varicella vaccination program. However, vulnerable populations remain and strategies to improve protection of particularly children <18 months of age and immunocompromised individuals, should be explored.