The impact of elevation and population density on dengue incidence and force of infection across the Philippines: Implications for climate-adapted surveillance.

Climate change is accelerating the geographical expansion of Aedes mosquito vectors, facilitating the emergence of arboviral diseases such as dengue in new regions. However, there is limited understanding of how environmental factors, such as temperature and population density, differentially influence key metrics of dengue transmission. This study investigates how elevation (a proxy for temperature) and population density affect dengue incidence and force of infection (FOI), both independently and dependently, across the Philippines, with implications for climate-adapted surveillance and control. We conducted a nationwide, cross-sectional survey across the Philippines (2013-2019), combining national dengue case data (N = 1,112,317) with antibody IgG seroprevalence data from a representative sample (N = 22,270). Dengue FOI (the predicted rate at which individuals become exposed to DENV annually) and incidence were estimated across elevation and population density strata using catalytic models. Regression analyses were used to assess interactions between geographic factors and the dengue FOI. Elevation and population density were associated with FOI, but not with reported dengue incidence, which varied unpredictably over space and time. Urban, low-elevation barangays had the highest FOI (13.2%), while rural and urban, high-elevation areas had lower FOI (<7.7%). These patterns held across years, island groups, and regions. A statistical interaction between elevation and population density on the dengue FOI improved model fit (p < 0.0053) and revealed population density is a driver of transmission intensity in low lying areas (<1200m) but not in high elevation areas (>1200m). Dengue burden in the Philippines is highest in low-lying, urban areas and significantly lower in high-elevation zones, irrespective of population density, where cooler temperatures prevail. FOI is a more reliable metric than incidence for understanding transmission dynamics and guiding interventions. Our findings highlight the need for geographically tailored control strategies, particularly as climate change alters environmental and demographic conditions.