Vax Populi : The Social Costs of Online Vaccine Skepticism

We quantify the effects of online vaccine skepticism on vaccine uptake and health complications for individuals not targeted by immunization campaigns. We collect the universe of Italian vaccine-related tweets for 2013-2018, label anti-vax stances using NLP, and match them with vaccine coverage and vaccine-preventable hospitalizations at the most granular level (municipal-ity and year). We propose a model of opinion dynamics on social networks that matches the observed data and shows that a vaccine mandate increases the average vaccination rate, but it also increases the controversialness around the topic, endogenously fueling polarization of opinions among users. We then leverage the intransitivity in network connections with “friends of friends” to isolate the exogenous source of variation for users’ vaccine-related stances and implement an IV strategy. We find that a 10pp increase in the municipality anti-vax stance causes a 0.43pp de-crease in coverage of the Measles-Mumps-Rubella vaccine, 2.1 additional hospitalizations every 100k residents among individuals untargeted by the immunization (newborns, the immunosup-pressed, pregnant women) and an excess expenditure of 7,311 euro, representing an 11% increase in health expenses