Communicating during a crisis can be challenging for public agencies as their communication ecology becomes increasingly complex while the need for fast and reliable public communication remains high. Using the lens of communication ecology, this study examines the online communication of national public health agencies during the COVID-19 pandemic in Italy, Sweden, and the United States. Based on content analysis of Twitter data ( n = 856) and agency press releases ( n = 95), this article investigates two main questions: (1) How, and to what extent, did national public health agencies coordinate their online communication with other agencies and organizations? (2) How was online communication from the agencies diversified in terms of targeting specific organizations and social groups? Our findings indicate that public health agencies relied heavily on internal scientific expertise and predominately coordinated their communication efforts with national government agencies. Furthermore, our analysis reveals that agencies in each country differed in how they diversify information; however, all agencies provided tailored information to at least some organizations and social groups. Across the three countries, information tailored for several vulnerable groups (e.g., pregnant women, people with disabilities, immigrants, and homeless populations) was largely absent, which may contribute to negative consequences for these groups.