Vulnerability of French overseas territories: the 2018-2022 hepatitis A outbreak in New Caledonia.

Background: Hepatitis A virus (HAV) typically causes asymptomatic infection, especially in children <6 y of age. New Caledonia has low HAV endemicity, but a surge in cases between 2018 and 2022 prompted an analysis of epidemiological and virological features.

Methods: A retrospective study analysed confirmed HAV cases (2018-2022), examining demographics, hospitalisations and alanine aminotransferase (ALT) levels. Phylogenetic analysis was conducted on isolated strains.

Results: In mid-2018, imported cases from Vanuatu triggered the outbreak. Higher incidence rates were observed in the Loyalty Islands, particularly Maré Island, with subsequent spread to other areas. From 2018 to 2022, 672 cases were identified, with a median age of 13 y. Children <6 y of age accounted for 8.9% of cases, those ages 6-14 y for 54.2% and individuals ≥15 y for 36.9%. Hospitalisation occurred in 32.3% of cases. ALT levels were higher in hospitalised patients (p<0.001), but children <5 y of age had lower ALT levels overall. HAV genotyping from 16 available sera revealed subgenotype IA strains, with 14 of them linked to Vanuatu.

Conclusions: The outbreak, triggered by imported strains from Vanuatu, was facilitated by environmental and infrastructural vulnerabilities. It highlights the need for improved vaccination, especially for travellers, and the critical role of water management in preventing future outbreaks. This also underscores the importance of tracking viral circulation in the Pacific region.