  Misinformation promotes distrust in science, undermines public health, and
may drive civil unrest. Vaccine misinformation, in particular, has stalled
efforts to overcome the COVID-19 pandemic, prompting social media platforms'
attempts to reduce it. Some have questioned whether "soft" content moderation
remedies -- e.g., flagging and downranking misinformation -- were successful,
suggesting that the addition of "hard" content remedies -- e.g., deplatforming
and content bans -- is necessary. We therefore examined whether Facebook's
vaccine misinformation content removal policies were effective. Here, we show
that Facebook's policies reduced the number of anti-vaccine posts but also
caused several perverse effects: pro-vaccine content was also removed,
engagement with remaining anti-vaccine content repeatedly recovered to
pre-policy levels, and this content became more misinformative, more
politically polarised, and more likely to be seen in users' newsfeeds. We
explain these results as an unintended consequence of Facebook's design goal:
promoting community formation. Members of communities dedicated to vaccine
refusal appear to seek out misinformation from multiple sources. Community
administrators make use of several channels afforded by the Facebook platform
to disseminate misinformation. Our findings suggest the need to address how
social media platform architecture enables community formation and mobilisation
around misinformative topics when managing the spread of online content.
