Epidemiological and virological assessment of influenza activity in Europe during the 2003-2004 season.

The 2003-2004 influenza season in Europe was dominated by the spread of the new drift variant A/Fujian/411/2002 (H3N2)-like virus which was not perfectly matched with the A(H3N2) component of the influenza vaccine. Sporadic cases of this virus were detected in Europe at the end of the 2002-2003 season and influenza activity associated with this virus began relatively early during the 2003-2004 season. Generally, influenza activity first occurred in the west of Europe (Ireland, the United Kingdom and the Iberian Peninsula) in October/November and gradually moved east across Europe, affecting Latvia, Lithuania and Poland during the months of January and February 2004. In general, the intensity of clinical activity was higher than during the 2002-2003 season (in 13 out of 20 networks) and, in countries reporting age specific data, the highest consultation incidences were observed among children aged 0-14. However, despite the emergence of the A(H3N2) drift variant, clinical incidences were not especially high compared with historical data. The composition of the 2004-2005 influenza vaccine has been modified compared with the 2003-2004 season and includes an A/Fujian/411/2002 (H3N2)-like virus strain and a new B virus strain (a B/Shanghai/361/2002-like virus).