Indicators of influenza activity
The levels of influenza activity in European countries reported by EISN members during the influenza season are based on three assessments:
- An indicator of the overall intensity of influenza activity in the country
- An indicator of the geographical spread of influenza in the country
- An indicator of trend in ILI/ARI sentinel consultations in the country compared to the previous week
Indicator of the overall intensity of influenza activity
The intensity of influenza activity is based on the overall level of clinical influenza activity in the country (or region), measured as ILI or ARI consultation rate. Each country assesses the intensity of clinical activity based on the historical data at its disposal . Some countries, for example the UK (England) and the Netherlands, have data covering more than 30 years.
Usually, there will be a 6–12 week period in winter when the level of clinical influenza activity rises above the baseline threshold, but in the very occasional winter activity never gets above the baseline level.EISN uses the following definitions to indicate the intensity of influenza activity in each country (or region):
Low: no activity or activity at baseline*
Medium: usual levels of activity
High: levels of activity higher than usual
Very high: exceptionally high levels of activity *Baseline influenza activity is the level at which clinical influenza activity remains throughout the summer and most of the winter.
Indicator of the geographical spread of influenza
Each country defines the geographical spread of influenza according to the definitions outlined below.
|Countries may be made up of one or more regions
|The population under surveillance in a defined geographical sub-division of a country. A region should not (generally) have a population of less than 5 million unless the country is large with geographically distinct regions
|Either no laboratory-confirmed cases, or no evidence of increased or unusual respiratory disease activity
|Isolated cases of laboratory-confirmed influenza infection
|Laboratory-confirmed influenza infection limited to one administrative unit in the country (or reporting site) only
|Laboratory-confirmed influenza infection appearing in multiple but under 50% of the administrative units of the country (or reporting sites)
|Laboratory-confirmed influenza infection appearing in 50% or more of the administrative units of the country (or reporting sites)
Indicator of the influenza trend
Trend is reported by the countries as increasing, stable or decreasing. Trend is a comparison of the level of ILI/ARI sentinel consultations during one week with the previous week.
Outside the influenza season, when ILI and ARI rates are at baseline level, increasing or decreasing trends are not informative.
Increasing: evidence that the level of respiratory disease activity is increasing compared to the previous week.
Stable: evidence that the level of respiratory disease activity is unchanged compared to the previous week.
Decreasing: evidence that the level of respiratory disease activity is decreasing compared to the previous week.