From week 40/2015, the start of weekly reporting on influenza activity in the WHO European Region, to week 13/2016, over 120 000 influenza detections across the Region have been reported. Influenza type A viruses are prevailing over type B but, unlike the situation in the 2014–15 season, A(H1N1)pdm09 viruses are prevailing over A(H3N2) and the proportion of B/Victoria-lineage detections has risen substantially, representing ~94% of those assigned to a lineage.
To date, 24 EU/EEA countries have shared 472 influenza-positive specimens with the Francis Crick Institute, London, for detailed characterisation: one additional country and 48 specimens since the February 2016 report. Since the latter report, 67 viruses have been characterised antigenically and genetic analyses are ongoing.
The 35 A(H1N1)pdm09 viruses characterised antigenically were similar to the vaccine virus A/California/7/2009. Worldwide, new genetic sub-clusters of viruses within the 6B clade have emerged, with two being designated as subclades: 6B.1 defined by HA1 amino acid substitutions S162N and I216T and 6B.2 defined by HA1 amino acid substitutions V152T and V173I. Of the 178 viruses characterised genetically for the 2015–16 season, 24 (13%) were clade 6B, 147 (83%) were subclade 6B.1 and seven (4%) were subclade 6B.2.
The six A(H3N2) test viruses characterised by haemagglutination inhibition (HI) assay were poorly recognised by reference antiserum raised against egg-propagated A/Switzerland/9715293/2013, the vaccine virus recommended for use in the 2015–16 northern hemisphere influenza season, despite at least three of the test viruses falling in the same subclade (3C.3a) as the vaccine virus. The test viruses were recognised somewhat better by antisera raised against egg-propagated A/Hong Kong/4801/2014, the virus recommended for use in 2016 southern hemisphere and 2016–17 northern hemisphere influenza vaccines. Of 55 A(H3N2) viruses characterised genetically for the 2015–16 season: one (2%) was clade 3C.3, 32 (58%) were subclade 3C.2a and 22 (40%) were subclade 3C.3a.
The 23 B/Victoria-lineage viruses were antigenically similar to tissue culture-propagated surrogates of B/Brisbane/60/2008. All 56 viruses characterised genetically for the 2015–16 season fell in genetic clade 1A as do recently collected viruses worldwide.
Three B/Yamagata viruses have been characterised since the previous report; all reacted well with post-infection ferret antiserum raised against egg-propagated B/Phuket/3073/2013, the recommended vaccine virus for the northern hemisphere 2015–16 influenza season and for quadrivalent vaccines in the 2016 southern hemisphere and 2016–17 northern hemisphere seasons. All 10 viruses characterised genetically for the 2015–16 season fell in genetic clade 3.