Guided by the integrative model of behavioral prediction and the social identity of deindividuation effects model, this study used an online experiment (N = 322) to test a moderated-mediation model that linked exposure to user comments posted to COVID-19 vaccine news stories and vaccine intentions. The study was conducted in Vietnam when the Delta variant of COVID-19 spread to the country and the efficacy and side effects of the COVID-19 vaccine were controversial in the news. Results showed that, compared with vaccine-hesitancy comments, vaccine-acceptance comments significantly increased vaccine intentions through injunctive norms, response efficacy, and attitudes. This indirect association was only significant among participants who perceived commenters as in-group members. Compared with vaccine-hesitancy comments, a mixture of comments that featured both acceptance and hesitancy significantly increased perceived norms, perceived efficacy, and attitudes. Further, comments significantly changed participants’ perceived norms, perceived efficacy, and attitudes compared with the news stories. Theoretical and practical implications are presented.