3%) and 397 were B/Yamagata-lineage viruses (47.7%). The analyses of influenza B viruses by HI assays continued to demonstrate that antisera raised in

ferrets infected with egg-grown B viruses may react poorly with cell-grown B viruses, prompting the extensive use of cell-grown viruses for antiserum production in ferrets for use in HI assays [8]. In addition, influenza B viruses often generate antisera with lower titres than those raised against influenza A viruses and some WHO CCs undertake additional boosting of ferrets, which can potentially broaden the cross-reactivity of the antibody responses. For the B/Victoria-lineage viruses collected from September 2012 to February 2013, the combined HI data from all Nutlin-3a in vitro WHO CCs showed approximately 11% of isolates to have reduced HI titres with post-infection ferret antiserum raised against B/Brisbane/60/2008, a previously this website recommended vaccine virus of the B/Victoria-lineage, or cell-propagated viruses genetically similar to it (Table 1). During

this period few differences were seen in HI reactivity (Table 4) or in antigenic maps created from these data (Fig. S6). The vast majority of HA genes from recent B/Victoria-lineage viruses fell into genetic group 1 represented by B/Brisbane/60/2008 with signature AA substitutions N75K, N165K and S172P in HA1 (Fig. 5). A high resolution tree constructed with HA sequences from 357 B/Victoria-lineage isolates collected through GISRS since February 2012 is shown in Fig. S7 and illustrates the high predominance of recent viruses in genetic group 1. Genetic subgroups within group 1, 1A and 1B, have been identified and are associated with the amino acid substitution L58P in HA1. The majority of viruses were in subgroup 1A with leucine at residue 58 of HA1. Some of the recent virus isolates, mainly from China, that fell into subgroup 1B had proline at residue 58 of HA1 and had NA genes from different groups of the B/Victoria lineage, namely HA genes from the B/Victoria-lineage

subgroup 1B and NA genes from HA group 4 viruses (HA-1B/NA-4) with these intra-lineage reassortant viruses having the additional AA substitutions K272Q, E320K, D384N and A465T (the latter change leading to the gain of a potential glycosylation site) in the NA compared with viruses that carried ADP ribosylation factor both the HA and NA genes of genetic group 4. Viruses in a third small cluster within subgroup 1A carried the HA1 AA substitution V146I. An additional cluster within subgroup 1A has undergone intra-lineage reassortment inheriting the NA gene from isolates similar to those in HA group 3 (HA-1A/NA-3, represented by B/Uruguay/12/2008), but with additional AA substitutions L73F, S397R, M375K and A389T in the NA and another intra-lineage reassorted group with V15I in the HA1. The latter circulated recently in North America, Japan and Europe (Fig. S7).