Impact of a single-dose program on varicella hospitalizations and infectious disease inequity in New Zealand: A retrospective cohort study

Abstract

Background: In July 2017, a fully funded single-dose varicella vaccine was introduced to Aotearoa New Zealand's (NZ) National Immunization Schedule. This study assessed the vaccine's impact on hospitalizations, complications, and health disparities.

Methods: A retrospective cohort study was conducted, analyzing varicella-related hospitalizations between 2007 and 2023 for NZ children aged 0-15 years, using national health databases. First varicella-related hospitalizations were identified by ICD-10-AM discharge codes. Incidence rates and subsequent varicella-related complications were compared across varicella vaccine funding periods: before funding (2007-2017), transition (2018-2019), and after funding (2020-2023).

Findings: There were 4,946 varicella hospitalizations with a 74% reduction in varicella-related admissions after funding compared to before funding (from 368 to 94 admissions per year). Associated complications decreased by 83%, with infective complications driving this. The median age of first varicella-related hospitalization was 2 years, with 73% of first admissions in children under 5 years. Children aged 1-4 years showed the greatest benefit (88% reduction), followed by infants under 1 year (76%) indicating indirect vaccine protection. Māori and Pacific children experienced significant reductions.

Interpretation: The introduction of varicella vaccine on the National Immunization Schedule significantly reduced hospitalizations with varicella and its complications. However, health inequities in immunization coverage remain with Māori and Pacific children disproportionately affected. Efforts to increase vaccination coverage, particularly among underserved populations, are critical to sustaining and enhancing these gains.