Investigation of a rise in newly acquired hepatitis B in a metropolitan setting: insights from whole genome sequencing

Hepatitis B virus (HBV) is a blood-borne virus that is nationally notifiable in Australia. Despite inclusion of the HBV vaccine in the childhood National Immunisation Program, local and overseas-acquired cases occur. An increase in newly acquired HBV cases in metropolitan Perth between 1 August 2023 and 31 July 2024 triggered an outbreak investigation. Epidemiological analyses of demographics, disease onset, and risk factors were combined with laboratory investigations, including whole genome sequencing (WGS) for genotyping and phylogenetic analysis. Twenty-one newly acquired cases were included, with three additional cases investigated. Most cases were aged over 30 years (n = 19, 79%) and reported casual unprotected sex (n = 18, 76%); some were born overseas (n = 8, 33%) or used intravenous drugs (n = 8, 33%). Four had received at least three HBV vaccines, although timing of doses varied. Of the 20 sequenced cases, 12 were locally acquired sub-genotype A2 HBV. Three sub-genotype A2 clusters and one sub-genotype B3 cluster were identified. This study is one of the first in Australia to investigate an HBV outbreak using WGS. Although genetic diversity within individual patient samples (quasispecies) complicated relatedness analyses and limited the assessment of transmission pathways, WGS complemented epidemiological methods as there were few identified epidemiological links. The increase in cases was likely due to coincident introductions of multiple HBV strains into Perth with some limited local transmission.