In the course of the 2015–2016 influenza season, over 139 000 influenza detections across the Region have been reported. Influenza type A viruses have prevailed over type B with A(H1N1)pdm09 viruses, greatly outnumbering A(H3N2) and B/Victoria-lineage detections representing two-thirds of the type B viruses assigned to a lineage.
Since 1 January 2016, EU/EEA countries have shared 596 influenza-positive specimens with the Francis Crick Institute, London, for detailed characterisation. Since the July report, 47 viruses have been characterised antigenically and genetically.
Of the 21 A(H1N1)pdm09 viruses characterised antigenically all (100%) were similar to the vaccine virus A/California/7/2009. Worldwide new genetic subclusters 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 493 viruses characterised genetically for the 2015–2016 season, 29 (6%) were clade 6B, 453 (92%) were subclade 6B.1, and 11 (2%) were subclade 6B.2.
The two A(H3N2) test viruses characterised by haemagglutination inhibition (HI) assay were poorly recognised by reference antiserum raised against the recommended 2015–2016 vaccine virus, egg-propagated A/Switzerland/9715293/2013. The test viruses were recognised somewhat better by antisera raised against egg-propagated A/Hong Kong/4801/2014, the virus recommended for use in 2016–2017 northern hemisphere and both 2016 and 2017 southern hemisphere influenza vaccines. Of 127 A(H3N2) viruses characterised genetically for the 2015–2016 season, two (2%) were clade 3C.3, 90 (71%) were subclade 3C.2a, and 35 (27%) were subclade 3C.3a.
The 20 B/Victoria-lineage viruses were antigenically similar to tissue culture-propagated surrogates of B/Brisbane/60/2008. All 184 viruses characterised genetically for the 2015–2016 season fell in genetic clade 1A, as do recently collected viruses worldwide.
Four B/Yamagata viruses have been characterised since the previous report. They 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 2016–17 northern hemisphere and both 2016 and 2017 southern hemisphere seasons. Of the 29 viruses characterised genetically for the 2015–2016 season, 28 fell in genetic clade 3 and one in clade 2.