Assessing the association between floods and dengue incidence in Vietnam: A time series study.

Background: Dengue incidence has increased significantly in recent decades, especially in areas undergoing rapid urbanization and frequent flooding. Floods can promote dengue transmission by creating favorable mosquito breeding sites, yet the timing and strength of this association remain unclear, particularly in Vietnam, where flood-prone areas overlap with a high dengue burden. Methods: This study assessed the relationship between flood exposure and dengue incidence across Vietnam and in Ho Chi Minh City (HCMC), a densely populated, flood-prone urban center. Time-series analyses were conducted using monthly data from 63 provinces (2011-2019) and weekly data from 24 districts in HCMC. Generalized additive mixed models with a negative binomial distribution were used, incorporating distributed lag linear functions to estimate cumulative and delayed effects. Analyses were stratified by season (rainy vs. dry). Results: At the national level, flooding was associated with a 17% higher dengue incidence (relative risk [RR] = 1.17; 95% confidence interval [CI] = 0.94, 1.47), with the strongest effect observed 2 months after flooding (RR = 1.11; 95% CI = 1.03, 1.20). Associations were more pronounced during the rainy season (RR = 1.65; 95% CI = 1.17, 2.32). In HCMC, floods were linked to a 10.4% increase in dengue cases (RR = 1.10; 95% CI = 1.03, 1.18), with effects peaking at week 9 (RR = 1.03; 95% CI = 1.02, 1.04). Seasonal analysis revealed short-term declines in risk during the rainy season and delayed increases during the dry season, peaking at week 9 (RR = 1.03; 95% CI = 1.01, 1.04). Conclusions: These findings highlight the delayed and seasonal nature of flood-related dengue risk and support the integration of flood metrics into early warning systems and climate-informed vector control strategies.