Vaccines on the move and the war on polio

The rising number of refugees and internally displaced people (IDPs) presents new challenges for vaccine distribution and the spread of diseases. How do forcibly displaced population inflows affect infectious diseases incidence in host communities? Can a policy intervention that vaccinates children during their migration mitigate the impacts? To answer these questions, we examine the Pakistani mass internal displacement from the conflict-affected Federally Administered Tribal Areas in 2008. Using a difference-indifferences approach, we compare new polio cases in districts near and far from the conflict zone before and after 2008. The spatial distribution of districts relative to the historical region of Pashtunistan allows us to design a sample of comparable units. We show that a standard deviation increase in predicted IDP inflow leads to a rise in the new polio cases per 100,000 inhabitants. Poorer vaccination levels among IDP compared to native children in host communities are one of the main mechanisms. Implementing a vaccination policy targeting IDP children during their migration journey helps bridge the vaccination gap, with important welfare implications.