Influenza surveillance in Europe. Comparing intensity levels calculated using the Moving Epidemic Method.

Abstract

Although influenza-like illnesses (ILI) and acute respiratory illnesses (ARI) surveillance are well established in Europe, the comparability of intensity among countries and seasons remains an unresolved challenge.

Weekly ILI and ARI incidence rates and proportion of primary care consultations were modeled in twenty-eight countries for the 1996/1997-2013/2014 seasons using the moving epidemic method (MEM). We calculated the epidemic threshold and three intensity thresholds, which delimit five intensity levels: baseline, low, medium, high and very high. The intensity of 2013/2014 season is described and compared by country.

The lowest ILI epidemic thresholds appeared in Sweden and Estonia (below 10 cases per 100,000) and the highest in Belgium, Denmark, Hungary, Poland, Serbia and Slovakia (above 100 per 100,000). The 2009/2010 season was the most intense, with 35% of the countries showing high or very high intensity levels. The European epidemic period in season 2013/2014 started in January 2014 in Spain, Poland and Greece. The intensity was between low and medium and only Greece reached the high intensity level, in weeks 7-9/2014. Some countries remained at the baseline level throughout the entire surveillance period.

Epidemic and intensity thresholds varied by country. ILI and ARI levels normalized by MEM in 2013/2014 showed that the intensity of the season in Europe was between low and medium in most of the countries. Comparing intensity among seasons or countries is essential for understanding patterns in seasonal epidemics. An automated standardized model for comparison should be implemented at national and international levels. This article is protected by copyright. All rights reserved.