Establishing Epidemic and Intensity Thresholds After a Major Change in Respiratory Virus Surveillance in Spain in 2020

Abstract

We estimated acute respiratory infections (ARIs) and influenza-like illness (ILI) thresholds for SiVIRA, a new integrated surveillance system set up in Spain in 2020.

We retrospectively extracted diagnostic codes from primary healthcare databases between Weeks 30/2010 and 39/2021, using methods similar to SiVIRA. Epidemic and intensity thresholds were estimated using the moving epidemic method (MEM).

The epidemic threshold for the 2024-2025 season was 400 cases per 100,000 persons for ARI and 36 for ILI, and medium, high, and very high intensity thresholds were 953, 1079, and 1139 for ARI and 167, 218, and 246 for ILI, estimating low intensity.

Reconstructing the historical series using electronic health records was possible and allowed the estimation of thresholds. Experience gained will guide the choice of MEM parameters in the future.