Online communities play an important role in spreading public discontent and could contribute to polarization. This study focuses on anti-vaccination views in the Netherlands, which have intensified during the COVID-19 pandemic. We examined the structure and development of five Dutch anti-vaccination Telegram groups and studied their interactivity and posting behaviour. Using group-based trajectory modelling, we examined the development of users’ posting behaviour in these groups. We find four posting trajectories across all five groups. A small group of users contributes the majority of posts. Overall, posting frequency declines over time and our results do not show evidence for a group of users whose posting frequency increases. This is taken to indicate that only a small group of users spread their anti-vaccination views through Telegram groups. While social media can reach a broad audience, most users are not necessarily engaged to also actively contribute to the online anti-vaccination community. ...