  Engagement with misinformation on social media poses unprecedented threats to
societal well-being, particularly during health crises when susceptibility to
misinformation is heightened in a multi-topic context. This paper focuses on
the COVID-19 pandemic and addresses a critical gap in understanding online
engagement with multi-topic misinformation at two user levels: news sharers who
share source news items on social media and post viewers who engage with online
news posts. To this end, we conduct a comprehensive analysis of 7273
fact-checked source news claims related to COVID-19 and their associated posts
on X, through the lens of topic diversity and conspiracy theories. We find that
false news, particularly when accompanied by conspiracy theories, exhibits
higher topic diversity than true news. At the news sharer level, false news has
a longer lifetime and receives more posts on X than true news. Additionally,
the integration of conspiracy theories is significantly associated with a
longer lifetime for COVID-19 misinformation. However, topic diversity has no
significant association with news sharer engagement in terms of news lifetime
and the number of posts. At the post viewer level, contrary to the news sharer
level, news posts characterized by heightened topic diversity receive more
reposts, likes, and replies. Notably, post viewers tend to engage more with
misinformation containing conspiracy narratives: false news posts that contain
conspiracy theories, on average, receive 40.8% more reposts, 45.2% more likes,
and 44.1% more replies compared to false news posts without conspiracy
theories. Our findings suggest that news sharers and post viewers exhibit
different engagement patterns on social media regarding topic diversity and
conspiracy theories, offering valuable insights into designing targeted
misinformation intervention strategies at both user levels.
