Anticipating influenza-like illness outbreaks via syndromic surveillance using over-the-counter drug sales and primary health care data

Effective pandemic preparedness relies on integrating diverse data sources for early outbreak detection. This study assessed whether over-the-counter (OTC) drug sales and primary health care (PHC) data could anticipate surges in respiratory-disease–related hospitalizations in Brazil. From November 2022 to June 2025, we analysed weekly time-series across 510 regions using a negative binomial autoregressive model within Statistical Process Control techniques. OTC data anticipated 56.6% of 746 hospitalization surges 1–3 weeks in advance, detected 9.5% concurrently, and missed 33.9%. PHC data anticipated 59.5%, detected 10.3% concurrently, and missed 30.2%. PHC data showed higher sensitivity and specificity than OTC (69.8% vs. 66.1%, and 49.5% vs. 47.8%). Performance varied regionally, and in 76.7% of regions, at least one stream showed high precision. These findings support the value of OTC drug sales, alongside PHC data, as early indicators of hospitalization surges in respiratory illness surveillance.