  The kick-off of vaccination campaigns in Europe, starting in late December
2020, has been followed by the online spread of controversies and conspiracies
surrounding vaccine validity and efficacy. We study Twitter discussions in
three major European languages (Italian, German, and French) during the
vaccination campaign. Moving beyond content analysis to explore the structural
aspects of online discussions, our investigation includes an analysis of
polarization and the potential formation of echo chambers, revealing nuanced
behavioral and topical differences in user interactions across the analyzed
countries. Notably, we identify strong anti- and pro-vaccine factions
exhibiting heterogeneous temporal polarization patterns in different countries.
Through a detailed examination of news-sharing sources, we uncover the
widespread use of other media platforms like Telegram and YouTube for
disseminating low-credibility information, indicating a concerning trend of
diminishing news credibility over time. Our findings on Twitter discussions
during the COVID-19 vaccination campaign in major European languages expose
nuanced behavioral distinctions, revealing the profound impact of polarization
and the emergence of distinct anti-vaccine and pro-vaccine advocates over time.
