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 to 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.