Public information campaigns often rely on credible expert messengers to influence people’s beliefs. We study how messenger identity and communication technology shape persuasion in a large-scale randomized experiment on vaccination attitudes among young adults in Greece. Participants receive an identical public-health message that varies only in who delivers it—a medical scientist or a social media influencer—and in how it is delivered—through immersive virtual reality or conventional video. Exposure raises trust in vaccines and trust in the healthcare system. Scientists are rated substantially more credible than influencers yet are no more persuasive on average; the two groups’ effectiveness rests on different characteristics—the scientists’ on perceived reliability and knowledgeability, the influencers' on personal appeal, prior familiarity, and reputation. We find that persuasion is greater under virtual reality, consistent with stronger engagement. Changes in beliefs persist for at least six months, and initially hesitant participants report greater willingness to vaccinate. A model in which engagement, prior uncertainty, and effective credibility enter persuasion multiplicatively rationalizes these patterns.