EISS : Levels of activity

The levels of influenza activity in European countries reported by EISS members during the 2007-2008 influenza season are based on two assessments of influenza activity:

1. An indicator of the overall intensity of influenza activity in the country;
2. An indicator of the geographical spread of influenza in the country.

The intensity of influenza activity is based on the overall level of clinical influenza activity in the country (or region). Each country assesses the intensity of clinical activity based on the historical data at its disposal. Some countries have historical data that date back over 30 years (e.g. England and the Netherlands), whilst others have data that date back over shorter periods of time (e.g. Ireland).

Some networks can establish numeric thresholds that define the different intensity levels of clinical influenza activity.

EISS uses the following definitions to indicate the intensity of influenza activity in each country (or region):

Low: no influenza activity or influenza activity is at baseline level*

Medium: level of influenza activity usually seen when influenza virus is circulating in the country based on historical data

High: higher than usual influenza activity compared to historical data

Very high: influenza activity is particularly severe compared to historical data

* Baseline influenza activity is the level that clinical influenza activity remains in throughout the summer and most of the winter. 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 (perhaps 1 in 10) activity never gets above the baseline level.

Each country defines the geographical spread of influenza according to the definitions outlined below. The definitions are based on those used by the WHO global influenza surveillance system - FluNet.

No report: no report received

No activity: reports indicate no evidence of influenza virus activity. Cases of ILI/ARI may be reported in the country but the overall level of clinical activity remains at baseline levels and influenza virus infections are not being laboratory confirmed. Cases occurring in people recently returned from other countries are excluded

Sporadic: isolated cases of laboratory confirmed influenza infection in a region, or an outbreak in a single institution (such as a school, nursing home or other institutional setting), with clinical activity remaining at or below baseline levels. Cases occurring in people recently returned from other countries are excluded

Local outbreak: increased ILI/ARI activity in local areas (such as a city, county or district) within a region, or outbreaks in two or more institutions within a region, with laboratory confirmed cases of influenza infection. Levels of activity in remainder of region, and other regions of the country, remain at or below baseline levels

Regional activity*: ILI/ARI activity above baseline levels in one or more regions with a population comprising less than 50% of the country's total population, with laboratory confirmed influenza infections in the affected region(s). Levels of activity in other regions of the country remain at or below baseline levels
* This term is not (generally) to be used in countries with a population of less than 5 million unless the country is large with geographically distinct regions

Widespread activity: ILI/ARI activity above baseline levels in one or more regions with a population comprising 50% or more of the country's population, with laboratory confirmed influenza infections

The figures below present the geographical spread of influenza in a visual format.

Last updated: 11 October 2006

ILI:	influenza-like illness
ARI:	acute respiratory infection
Country:	countries may be made up of one (e.g. the Netherlands) or more regions (e.g. England South, Central and North)
Region:	the population under surveillance in a defined geographical sub-division of a country (e.g. England South, Central and North). A region should not (generally) have a population of less than 5 million unless the country is large with geographically distinct regions