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Comparative Study
. 2015 Sep;9(5):234-46.
doi: 10.1111/irv.12330.

Influenza surveillance in Europe: comparing intensity levels calculated using the moving epidemic method

Tomás Vega  1 , José E Lozano  1 , Tamara Meerhoff  2 , René Snacken  3 , Julien Beauté  3 , Pernille Jorgensen  4 , Raúl Ortiz de Lejarazu  5 , Lisa Domegan  6 , Joël Mossong  7 , Jens Nielsen  8 , Rita Born  9 , Amparo Larrauri  10 , Caroline Brown  4
Affiliations
Comparative Study

Influenza surveillance in Europe: comparing intensity levels calculated using the moving epidemic method

Tomás Vega et al. Influenza Other Respir Viruses. 2015 Sep.

Abstract

Objectives: 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. The objective is to compare the intensity of ILI and ARI in some European countries.

Design and setting: Weekly ILI and ARI incidence rates and proportion of primary care consultations were modeled in 28 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.

Results: 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 to 9/2014. Some countries remained at the baseline level throughout the entire surveillance period.

Conclusions: Epidemic and intensity thresholds varied by country. Influenza-like illnesses 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.

Keywords: Incidence; influenza-like illnesses; primary care; surveillance.

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Figures

Figure 1
Figure 1
MEM graph model with epidemic and intensity thresholds, intensity levels, and the weekly ILI/ARI rate.– weekly rate > very high intensity threshold.– High intensity threshold < weekly rate ≤ very high intensity threshold.– medium intensity threshold < weekly rate ≤ high intensity threshold.– epidemic threshold < weekly rate ≤ medium intensity threshold.– weekly rate ≤ epidemic threshold. ILI, influenza-like illnesses; ARI, acute respiratory illnesses; MEM, moving epidemic method.
Figure 2
Figure 2
(A) Intensity levels and highest weekly ILI rate by country in the 2013/2014 season. (B) Intensity levels and highest weekly ARI rate by country in the 2013/2014 season. ILI, influenza-like illnesses; ARI, acute respiratory illnesses.
Figure 3
Figure 3
Weekly ILI/ARI intensity levels in Europe 2013/2014 during the epidemic period (weeks 01/2014 to 15/2014). ILI, influenza-like illnesses; ARI, acute respiratory illnesses.
See this image and copyright information in PMC

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