Sentinel surveillance

The clinical surveillance of influenza in the European Influenza Surveillance Network (EISN) is generally based on reports made by sentinel general practitioners. Some of the sentinel surveillance systems also include paediatricians and physicians with other specialisations. The physicians usually represent 1-5% of physicians working in the country or region.

Most sentinel surveillance systems report data on the number of new cases of influenza-like illness (ILI) or acute respiratory infection (ARI). Some networks report both ILI and ARI.

Reporting time schedule

Surveillance data is reported weekly on Flu News Europe, a collaboration between ECDC and the WHO Regional Office for Europe. The report is based on the data collected by sentinel physicians and reported by the nominated national operational contact points for influenza surveillance to The European Surveillance System (TESSy) at ECDC. Influenza activity is monitored throughout the year in Europe but more intensely from early October (week 40) in one year to the end of May (week 20) the following year.

Laboratory surveillance of influenza

Surveillance of influenza virus infection on a Europe-wide level requires close collaboration between virologists, epidemiologists and sentinel GP networks to generate the data necessary to inform a timely public health response.

Physicians in sentinel networks may take nose and/or throat swabs from a subset of patients with influenza-like illness (ILI) or acute respiratory infection (ARI). Some sentinel surveillance systems also collect blood samples. The specimens are then sent to the national reference laboratory and are tested for influenza viruses. For positive samples, influenza subtypes are determined. These results are used to validate the clinical reports of ILI and ARI.

The national reference laboratories also report influenza test results from non-sentinel surveillance to EISN. Specimens (nose swabs, throat swabs and blood samples) for non-sentinel surveillance can come from a wide range of sources: hospitals, non-sentinel physicians, homes for the elderly, clinics, etc.

A subset of the detected viruses is antigenically and genetically characterised. In addition, the relatedness between the circulating and vaccine strains is evaluated.

Analysis of resistance against neuraminidase inhibitors and adamantanes is also performed by measuring 50% inhibitory concentration (IC50) values, and/or by genotyping of viruses for detection of known drug resistance mutations. In Europe antiviral resistance is monitored by ECDC and WHO based on reports sent by influenza reference laboratories to The European Surveillance System (TESSy).

Data from laboratory surveillance are included in the weekly surveillance updates published on Flu News Europe.

Hospital-based surveillance

Some countries conduct surveillance for hospitalised cases presenting with severe acute respiratory infection (SARI). Depending on the country, all or a subset of SARI patients are tested for influenza virus infection.

A subset of countries reports laboratory-confirmed influenza-positive hospitalized cases every week. Depending on the country, these cases are from intensive care units (ICU) and/or other wards.

Case definitions, populations under surveillance, and data formats differ among the countries.

Data from hospital-based surveillance are included in Flu News Europe.

Mortality surveillance

Surveillance of all-cause mortality in at least 15 European countries is carried out by the EuroMOMO network. EuroMOMO also has a project called FluMOMO that estimates mortality that could be ascribed to influenza.