  We use aggregated data from Facebook to show that COVID-19 is more likely to
spread between regions with stronger social network connections. Areas with
more social ties to two early COVID-19 "hotspots" (Westchester County, NY, in
the U.S. and Lodi province in Italy) generally had more confirmed COVID-19
cases by the end of March. These relationships hold after controlling for
geographic distance to the hotspots as well as the population density and
demographics of the regions. As the pandemic progressed in the U.S., a county's
social proximity to recent COVID-19 cases and deaths predicts future outbreaks
over and above physical proximity and demographics. In part due to its broad
coverage, social connectedness data provides additional predictive power to
measures based on smartphone location or online search data. These results
suggest that data from online social networks can be useful to epidemiologists
and others hoping to forecast the spread of communicable diseases such as
COVID-19.
