  In this paper, we study the behavior of users on Online Social Networks in
the context of Covid-19 vaccines in Italy. We identify two main polarized
communities: Provax and Novax. We find that Novax users are more active, more
clustered in the network, and share less reliable information compared to the
Provax users. On average, Novax are more toxic than Provax. However, starting
from June 2021, the Provax became more toxic than the Novax. We show that the
change in trend is explained by the aggregation of some contagion effects and
the change in the activity level within communities. In fact, we establish that
Provax users who increase their intensity of activity after May 2021 are
significantly more toxic than the other users, shifting the toxicity up within
the Provax community. Our study suggests that users presenting a spiky activity
pattern tend to be more toxic.
