  We analyze data from Twitter to uncover early-warning signals of COVID-19
outbreaks in Europe in the winter season 2019-2020, before the first public
announcements of local sources of infection were made. We show evidence that
unexpected levels of concerns about cases of pneumonia were raised across a
number of European countries. Whistleblowing came primarily from the
geographical regions that eventually turned out to be the key breeding grounds
for infections. These findings point to the urgency of setting up an integrated
digital surveillance system in which social media can help geo-localize chains
of contagion that would otherwise proliferate almost completely undetected.
