Applying weighted quantile sum regression to wastewater surveillance: enteric viral mixtures and diarrheal disease trends in Costa Rica.

Wastewater surveillance is a valuable tool to track viral circulation in populations; however, its application in tropical settings remains limited. In this study, we implemented a year-round wastewater surveillance program in a great metropolitan area of Costa Rica to assess its feasibility and epidemiological utility. Four major enteric viruses: rotavirus (RoV), norovirus genogroups I (NVG1) and II (NVG2), and pan-enterovirus (PEV), were quantified by RT-qPCR in the effluent of the country's main wastewater treatment plant. Weighted Quantile Sum regression was applied to compare the mixture of viruses detected with reported diarrhoea cases during the study period (June 2023- July 2024). NVG2 was the most frequently detected virus, while RoV and PEV peaked during the rainy season. The WQS mixture index showed no significant association with weekly case counts at any lag (RR 0.928-1.091; all p > 0.05). Component weights varied by model, and the rainy season was the only factor significantly associated with higher case counts (RR = 1.42, p = 0.012). These findings demonstrate that wastewater surveillance is feasible in tropical contexts and can provide complementary insights to traditional systems, although temporal and seasonal structure strongly modulate observed associations.