Weekly ILI and ARI incidence rates or proportions of primary care consultations from the 1996/1997 to the 2013/2014 influenza season (from week 40 to week 20 of the next year) were taken from the EuroFlu database (WHO Regional Office for Europe). Countries included in the study were selected among the 50 participating Member States in the WHO European Region, according to the following criteria: data available for at least six consecutive seasons, excluding the pandemic season 2009/2010 (a minimum of five seasons for the calculations and the target season), and no major changes in the surveillance systems during the reporting period. Countries fulfilling these criteria were invited to participate.
Data were checked for inconsistencies, such as abnormal weekly estimates or missing values during the surveillance period, and sent to the country representatives for validation and updating when necessary.
A sequential analysis using the R Language implementation of MEM (package “mem” [Internet]. Available from:
http://cran.r-project.org/web/packages/mem/index.html) was carried out for each country to calculate the epidemic threshold (level of influenza activity that signals the start and end of the annual epidemic wave) and the three intensity thresholds (medium, high, and very high) for each season. The number of seasons included in each analysis ranged from five to 10 (training period).
Moving epidemic method has three main steps, which have been previously described.14 (
link) In the first step, for each season separately, the length of the epidemic period is estimated as the minimum number of consecutive weeks with the maximum accumulated percentage rates, splitting the season in three periods: a pre-epidemic, an epidemic, and a post-epidemic period. In the second step, MEM calculates the epidemic threshold as the upper limit of the 95% one-sided confidence interval of 30 highest pre-epidemic weekly rates, the
n highest for each season taking the whole training period, was
n = 30/number of seasons. In the third step, medium, high, and very high intensity thresholds were estimated as the upper limits of the 40%, 90%, and 97·5% one-sided confidence intervals of the geometric mean of 30 highest epidemic weekly rates, the
n highest for each season taking the whole training period, were
n = 30/number of seasons. For the purposes of this work, if the medium intensity threshold is lower than the epidemic threshold, the epidemic threshold is used for both.
The intensity levels were defined as follows (Figure
1):
For the analysis, at least 5 years of consecutive data were required to calculate the threshold and intensity levels for the next season; that is, the analysis started with data from seasons 1996/1997 to 2000/2001 to estimate the thresholds for season 2001/2002, or from the first season available in the country to the fifth one, to estimate the thresholds for the sixth season. From then on, calculations for each subsequent season included one more season of data (to a maximum of 10) to estimate the thresholds. The last step of analysis used data from a maximum of 10 seasons if available (2002/2003 to 2012/2013, excluding the 2009 A(H1N1) pandemic) or at least five seasons (2007/2008 to 2012/2013, excluding the pandemic) to make estimations for season 2013/2014. As we excluded the pandemic season from MEM calculations, estimations for the 2009/2010 and 2010/2011 season thresholds are the same.
For each country, the highest weekly rate per season (the season peak) was compared to the intensity thresholds and described for each of the countries over time. Furthermore, a log scale of the weekly incidence rates and percentage of consultations was used to graphically compare and discuss the season 2013/2014 country intensity levels in Europe.
Finally, weekly maps were drawn to show the spread of the 2013/2014 season intensity in Europe.
Complementary figures in
Appendix S1 show the historical data included in this study, the threshold trend over the years, and the season 2013/2014 surveillance.
Appendix S2 is an animated gif of the evolution of the 2013/2014 intensity levels by country.
The R Language (v3.2.0) mem library (v1.4) was used for calculations of the thresholds and graphic output.
Vega T., Lozano J.E., Meerhoff T., Snacken R., Beauté J., Jorgensen P., Ortiz de Lejarazu R., Domegan L., Mossong J., Nielsen J., Born R., Larrauri A, & Brown C. (2015). Influenza surveillance in Europe: comparing intensity levels calculated using the moving epidemic method. Influenza and Other Respiratory Viruses, 9(5), 234-246.