In itinere infections covertly undermine localized epidemic control in metapopulations.

Metapopulation models have traditionally assessed epidemic dynamics by emphasizing local (in situ) interactions within defined subpopulations, often neglecting transmission occurring during mobility phases (in itinere). Here, we extend the Movement-Interaction-Return metapopulation framework to explicitly include contagions acquired during transit, considering agents traveling along shared transportation networks. We reveal that incorporating in itinere contagion entails a notable reduction of the epidemic threshold and a pronounced delocalization of the epidemic trajectory, particularly significant in early-stage outbreaks.