selected according to the following criteria:

1 ILI was registered independently from ARI.2 Historical data were available for five or more surveillance seasons.3 Weekly rates were estimated at least for 33 weeks in every season, from week 40 of 1 year to the week 20 of the following year. For the 2009--2010, the season the period was extended from the 30th week of 2009 to the 20th week of 2010 because of the circulation of the influenza A (H1N1)pdm09 virus.4 The number of sites reporting data and the population under surveillance remained stable over time.

Data quality control {#ss4}
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Missing values in weekly rates at the beginning or at the end of the surveillance period did not affect the method and were excluded from calculations. Missing weekly data during the surveillance period were imputed with a smooththe number of weeks between the alert week and the first week of the epidemic period modelled by MEM for the 2009--2010 season, either the alert week precedes the epidemic period or *vice versa*.

Cross‐validation analysis {#ss7}
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The accuracy of the method was evaluated using a cross‐validation procedure. For each country, each single season was extracted from the historical records to be used as the target season. MEM calculated the beginning and the end of the epidemic period of the target season. On the basis of the remaining seasons, excluding the target season, MEM calculated the pre‐ and post‐epidemic thresholds. This procedure was repeated as often as the numbers of seasons available per country.

To assess the quality of the method, the rates of the target season inside and outside of