Weekly influenza surveillance overview, 2 March 2012 - week 8

The national influenza season epidemics in Europe are continuing upwardsin a number of countries although they have peaked in at least two countries that were affected early. The epidemics remain dominated by A(H3) viruses, but B viruses seem to be become more important. This is reported in the latest edition of the Weekly Influenza Surveillance Overview (week 8/2012).

Like last week, medium or high intensity was reported by 17 countries; 13 countries reported increasing trends. Bulgaria and Italy have reported decreasing trends for three weeks in a row and thus are the first two countries that have peaked in Europe this season.

Virological surveillance for the period shows that out of 1 712 sentinel specimens tested, 884 (51.6%) were positive for influenza virus, a similar percentage as in the two previous weeks. Of these sentinel viruses, 88.9% were type A and 11.1% were type B. This is the highest percentage for B this season. More details on the antigenic and genetic characteristics of circulating viruses in Europe can be found in the latest Influenza virus characterisation report, published in February 2012.

Further, a published analysis of severe influenza cases in France, Ireland, Spain and the United Kingdom found that the 2011–12 season has so far been dominated by influenza A(H3), but hospitalised cases in those four countries showed a relatively greater proportion of influenza A(H1N1)pdm09 viruses.

The formal WHO review of circulating viruses this season found that influenza A(H3) and B viruses have moved genetically and antigenically away from the 2011–2012 seasonal vaccine viruses. This prompted WHO to recommend different vaccine viruses for the 2012–2013 seasonal vaccine. See more details in the ECDC article posted on 1 March 2012: WHO recommendation on influenza virus vaccines for the northern hemisphere 2012-2013 season