The 2004-2005 influenza season in Europe and the emergence of a new influenza A (H3) virus strain.

Background: The European Influenza Surveillance Scheme (EISS) is a European Union (EU) Designated Surveillance Network which is partly funded by the European Commission. An important objective of EISS is to provide timely and authoritative clinical, epidemiological, and virological information on influenza activity in Europe. Methods: Twenty-three countries actively participated in EISS (www.eiss. org) during the 2004–2005 influenza season (20 EU countries, plus Norway, Romania, and Switzerland). Clinical data are collected by sentinel physicians in each country and virological testing is carried out by national reference laboratories. Results: Clinical influenza activity varied across Europe during the 2004– 2005 season and in a number of countries (e.g. The Netherlands, Slovenia, and Spain) the peak was higher than in the previous four seasons. There was a general west–east spread of influenza activity and clinical incidences were highest in the 0–4 and 5–14 age groups. A total of 14 183 influenza virus detections were reported (data up to week 16/2005): 11 839 (83%) were influenza A [of which 38% were A(H3)] and 2344 (17%) were influenza B. Influenza B detections gradually increased and were dominant at the end of the season. A total of 4083 viruses were antigenically characterized and 31% (1263) were antigenically closely related to the new reference strain A/California/7/ 04 (H3N2) announced by WHO in February 2005. The proportion of A/California/7/04 (H3N2)-like viruses increased (it was 17% in week 15/2005), as isolates were reclassified using the new WHO reference reagents for A/California/7/04 (H3N2). Conclusions: EISS is able to provide real-time data on influenza activity in Europe. The new, non-vaccine, A/California/7/04 (H3N2)-like virus played an important role in the influenza epidemic and its full impact will be assessed when complete virological data are available. (aut.ref.