Interventions with high levels of engagement have been found to be more effective at changing health behavior than those with low levels. Few prior studies have examined whether engagement in interventions using social media groups is associated with higher program effectiveness. This study examined whether participant engagement in a moderated private Facebook group aimed at COVID-19 vaccination was associated with improvements in immunization intentions (N = 263). Engagement in the group (i.e. emoji reactions, comments, poll votes and posts) and change in COVID-19 vaccine intentions were assessed through a retrospective review of participant engagement metrics and self-reported surveys at 4 weeks. Overall, 59% of participants engaged at least once. Participants on average made 11.9 total engagements during the 4-week study (SD = 25.5), of which 6.3 (SD = 15.2) were emoji reactions, 4.0 (SD = 12.3) comments, 1.4 (SD = 2.2) poll votes and 0.2 (SD = 0.7) post. No significant differences were found between engagement and sociodemographic characteristics or group size. Emoji reactions were associated with increased intentions. Each emoji reaction used was associated with a 4% increase in the odds of improvements in intention to vaccinate adjusted odds ratio (aOR) = 1.04, 95% CI: 1.01–1.08, P = 0.015). Future interventions on social media should encourage users to use emoji reactions on posts.