  Vaccines were critical in reducing hospitalizations and mortality during the
COVID-19 pandemic. Despite their wide availability in the United States, 62% of
Americans chose not to be vaccinated during 2021. While online misinformation
about COVID-19 is correlated to vaccine hesitancy, little prior work has
explored a causal link between real-world exposure to antivaccine content and
vaccine uptake. Here we present a compartmental epidemic model that includes
vaccination, vaccine hesitancy, and exposure to antivaccine content. We fit the
model to observational data to determine that a geographical pattern of
exposure to online antivaccine content across US counties is responsible for a
pattern of reduced vaccine uptake in the same counties. We find that exposure
to antivaccine content on Twitter caused about 750,000 people to refuse
vaccination between February and August 2021 in the US, resulting in at least
29,000 additional cases and 430 additional deaths. This work provides a
methodology for linking online speech to offline epidemic outcomes. Our
findings should inform social media moderation policy as well as public health
interventions.
