Prediction markets (e.g., Polymarket, Kalshi) allow participants to bet on future events, producing real-time forecasts based on collective judgment. In domains such as elections and finance, markets have been effective at aggregating information, often rivaling or outperforming expert forecasters or polls. Whether this performance extends to infectious disease dynamics is unclear. Participants are self-selected and typically lack epidemiological expertise. However, markets can respond in real time to emerging news and unstructured signals in ways that standard forecasting pipelines cannot. Also, substantial financial stakes encourage participants to make an effort to be accurate. We evaluate Polymarket forecasts during 2025 and 2026 for two settings: weekly cumulative influenza hospitalizations in the US, which have an established expert-curated forecasting ensemble (CDC FluSight), and monthly measles cases, which do not. Across both settings, prediction markets fail to outperform standard benchmarks. For influenza, markets are competitive with low-performing individual FluSight models but are dominated by the FluSight ensemble: even when we combine market forecasts with the ensemble, the best combination puts zero weight on the markets. For measles, markets are outperformed by simple statistical baselines. We diagnose two sources of market inefficiency: placement of probability mass on impossible outcomes (e.g., decreasing values in cumulative forecasts) and low trading volume. These results suggest that current prediction markets are not reliable forecasters of infectious disease dynamics on their own or useful as complementary features for existing forecasting systems.

翻译：预测市场（如Polymarket、Kalshi）允许参与者对未来事件下注，从而基于集体判断生成实时预测。在选举和金融等领域，市场在信息聚合方面表现出色，常常与专家预测或民调相媲美甚至超越它们。然而，这种表现在传染病动态预测中是否同样有效尚不明确。参与者是自我选择的，通常缺乏流行病学专业知识。尽管如此，市场能够以标准预测流程无法实现的方式实时响应新兴新闻和非结构化信号。此外，巨大的经济利益激励参与者努力做出准确预测。我们评估了2025年和2026年Polymarket在两种场景下的预测：美国每周累计流感住院人数（已有专家策划的预测集成系统CDC FluSight）和每月麻疹病例（尚无此类系统）。在这两种场景中，预测市场均未能超越标准基线。对于流感，市场虽可与表现较差的单个FluSight模型竞争，但被FluSight集成系统全面压制：即使我们将市场预测与集成系统结合，最优组合方案赋予市场的权重为零。对于麻疹，市场表现甚至逊于简单统计基线。我们诊断出市场低效的两个根源：将概率质量分配给不可能的结果（例如累计预测中的递减值）以及低交易量。这些结果表明，当前的预测市场既无法独立作为可靠的传染病动态预测工具，也无法作为现有预测系统的有效补充特征。