Interdiscursivity and its historical shifts in China's public health risk communication letters: from SARS to the post-COVID-19 era.

Interdiscursivity refers to the blending of multiple discourses, genres, or styles within a single communicative event. This study investigates the interdiscursivity of Chinese public health risk communication letters issued during major health crises over the past two decades, including SARS, H1N1, COVID-19 and seasonal influenza. It examines how multiple discourse types-advice-giving, scientific, literary, authoritative, affective and informal-are interwoven and strategically adapted across different crises to guide public behaviour, mobilise communities and foster trust. Using qualitative document analysis combined with thematic analysis, the study traces the historical evolution of these interdiscursive patterns and situates them within China's broader public health documentary heritage. The findings demonstrate that these letters not only translate abstract risks into actionable guidance but also serve as instruments for documenting sociocultural shifts and governance practices, and for supporting collective memory and institutionalised learning. By linking the diachronic evolution of interdiscursive strategies to the social and documentary functions of public health communication, this study highlights the dual role of risk communication letters as both practical tools and enduring public health documents, offering insights for future communication strategies in health crises.