Association of typhoid fever with floods under climate variability in 82 Belt and Road Initiative countries (2000-2021): A mixed-effects model and implications for water and sanitation infrastructure.

Using four hierarchical mixed-effects models (MEMs), country-level data were analyzed to quantify longitudinal associations between floods and typhoid in 82 Belt and Road Initiative (BRI) countries (BRCs) under climate variability, adjusting for water, sanitation, hygiene (WASH), and population dynamics (2000-2021). Typhoid incidence (per 100,000) was the outcome variable, with flooding (occurrence, duration, and affected population), climate (including precipitation, temperature, and humidity), and covariates (drinking water, sanitation services, open defecation, and population) as predictors. Data sources included the Global Burden of Disease (typhoid), EM-DAT and FloodBase (floods), Copernicus ERA5 and Global Data Lab (climate), and World Bank and UNICEF (WASH and population). Model 2 (flood + climate variables) emerged as optimal for capturing flood-typhoid associations in BRCs, revealing climate-mediated pathways. Prolonged floods (>30 days) increased typhoid risk in vulnerable BRCs (like Bangladesh: β = 0.0021, *p < 0.001) by contaminating water and disrupting sanitation. Arid regions like the Middle East and North Africa showed null or inverse relationships due to short flood duration and pathogen die-off in dry conditions. Temperature consistently suppressed risk (like Kenya: β = -0.91, *p < 0.001), while precipitation had dual roles: dilution, which reduced risk in humid zones (like Vietnam: β = -0.0010, *p < 0.001) but increased concentrated contaminants in arid countries (like Chad: β = 0.0037, *p < 0.001). WASH covariates were often eclipsed by climate-flood interactions in Model 2. Riverine and flash floods posed higher risks than coastal floods. High-income BRCs exhibited negligible associations, attributable to robust infrastructure. Typhoid risk in most BRCs is predominantly mediated by climate-flood interactions, with Model 2 robustly capturing this pathway. Targeted BRI interventions, including climate- and flood-resilient WASH infrastructure and scaling China's "Toilet Revolution"-are critical for typhoid control in high-burden BRCs.