Structural fragility in digital public health communication.

Background: Digital public health campaigns often report robust reach and engagement metrics, yet these metrics obscure deeper structural vulnerabilities. We define structural fragility as the declining capacity of actors to maintain network visibility in competitive digital communication environments, and assess it in Twitter/X discourse on U.S. e-cigarette prevention (2014-2023). Methods: Accounts were classified into five categories (Health, Commercial, Vape Community, Organic, Other). Longitudinal network analysis tracked three dimensions of structural fragility: positional (amplification and centrality), community (fragmentation and homophily), and core (k-core presence). Annual directed retweet networks were constructed, and modularity, in-degree, PageRank, eigenvector centrality, and k-core composition were computed. Regression models were adjusted for the number of participating accounts. Results: Here we show the network contracted from 36,204 accounts in 2014 to 800 in 2022, a 97.8% reduction. It fragmented into increasingly homophilous clusters (modularity 0.16 to 0.81; within-category retweeting 6.7% to 42.6%). Health accounts fell from 59.1% of the deepest k-core in 2014 to 3.8% in 2015, as Vape Community accounts rose to 78.8%. Per-capita amplification declined 95.7%, and Health PageRank share fell from 0.41 to 0.34, with amplification concentrated among a few accounts (eigenvector Gini ≥ 0.93). These declines persisted after adjustment for account numbers. Conclusions: Health actors remained active while losing structural position, a "bunker effect" of broadcasting without amplification. These findings challenge exposure-based evaluation models. Core embedding, within-category concentration, and PageRank share are computable from public network data and offer practical measures for campaign evaluation and platform transparency.