Respiratory Disease Spectra Across Rainfall Regimes and Climate Belts: An Ecological Analysis of 204 Countries Combining GBD 2023 with WHO Influenza Surveillance Seasonality

Background: Rainfall seasonality is a candidate driver of respiratory disease, yet its global relationship with the respiratory disease spectrum across climate belts is unquantified. We tested a “dual-engine” hypothesis of a temperature-driven viral engine and a humidity-modulated bioaerosol engine, weighted by climate belt.Methods: Ecological analysis of 204 countries and territories combining Global Burden of Disease 2023 age-standardised rates for four respiratory causes, WHO FluNet influenza seasonality (150 countries) and WorldClim 2.1 rainfall (167 countries); belt-level analyses use the 200 countries with climate-belt assignment. Seasonal peaks were defined as circular weighted-mean angles. Influenza–rainfall alignment (offset between peaks, 0–6 months) was modelled with robust regression and, given spatial clustering, a spatial-error model. Engine contributions used standardised coefficient ratios and Shapley decomposition.Findings: Lower respiratory infections comprised 1.94% (Tropical) and 2.03% (Subtropical) of respiratory incidence versus 0.58% (Temperate). Median alignment was 2.90 months; 52.5% of countries co-peaked within three months. Adjusted offsets were 2.13 (Subtropical) and 1.42 (Tropical) months tighter than in the Temperate belt; the Tropical contrast attenuated under spatial-error modelling (p = 0.21). The negative temperature×humidity interaction was 0.72 (lower respiratory infections) and 1.13 (upper respiratory infections) times the temperature main effect.Interpretation: Rainy-season respiratory risk is structured by climate belt in a manner consistent with two spatially differentiated engine.