Based on dialogic and protection motivation theories, this study investigates the impact of governmental communication on publics’ vaccine advocacy on social media in the era of COVID-19 pandemic. Results of two online surveys at two different time points that include publics who are not vaccinated (Survey 1) and vaccinated (Survey 2) suggested that dialogic government communication fosters trust among the public, which in turn, influences their perceived threats of COVID-19 and efficacy in receiving the vaccine. Efficacy was positively associated with vaccine advocacy on social media for publics who are not vaccinated during the time when the vaccine was not widely available, whereas perceived threats were significantly related to vaccine advocacy for those vaccinated at the later stage of the pandemic. Theoretical and practical implications for the disciplines of communication and public relations are discussed.