Influence of Both La Nina and Island Isolation During COVID-19 on the Epidemiology of Infectious Diseases in New-Caledonia

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Background: New Caledonia, an archipelago in the South Pacific, experienced an unprecedented con-junction of prolonged border closure during the COVID-19 pandemic (2020 to 2022) and maked influence of the El Niño/Southern Oscillation (ENSO). This context provided a unique opportunity to explore how environmental drivers, island isolation, and so-cio-demographic factors interact to shape infectious disease dynamics. This study aimed to assess the respective and combination of climatic variability, travel restrictions, and so-cio-demographic factors on the dynamics of four priority infectious diseases. Methods: We analysed retrospectively data from 2017 to 2023 on four infectious diseases: leptospi-rosis, dengue, influenza, and hepatitis A (HAV). Satellite precipitation data and the Mul-tivariate El Niño/Southern Oscillation Index (MEI) were used. Socio-demographic and economic variables were gathered. Statistical analyses employed descriptive analysis, General Additive Mixed Models to evaluate the associations between climatic events, travel restrictions, and disease circulation using the communal level as random effect and time (daily) as spline effect. Results: We analysed: 878 cases of leptospirosis, 162 of HAV, 7,377 of influenza, and 6,607 dengue cases. Influenza was associated with rainfalls before lockdown (Odds Ratio (OR) 0.7, Con-fidence interval 95%, (CI95%), (0.6 - 0.8)), disappeared during lockdown but resurged post-reopening losing its meteorological association. Dengue epidemics declined, coin-ciding with Wolbachia program and border closure, and was associated with lower MEI (OR 0.78, CI95% (0.6-1) during the 2017 to 2020 period. HAV cases were correlated with the MEI (OR: 1.8, CI95% (1-3.3)). Leptospirosis cases were associated with cumulative rainfall (OR 1.12 (1.1-1.2), lower education (OR 1.04, CI95% (1-1.1)), and decreased with water supply (OR 0.7, CI95% (0.5-0.8)). Conclusion: Our findings highlight how climate variability, mobility restrictions, and so-cio-environmental inequities differentially shape infectious disease risks in island ecosys-tems. These results reinforce the need for integrated One Health surveillance that jointly addresses environmental change, social vulnerability, and infectious disease prevention.

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