CDC has antigenically or genetically characterized 400 influenza viruses collected during October 1 December 9, 2017, and submitted by U.S. laboratories, including 58 influenza A(H1N1)pdm09 viruses, 256 influenza A(H3N2) viruses, and 86 influenza B viruses.
A (H1N1)pdm09: Phylogenetic analysis of the HA genes from 58 A(H1N1)pdm09 viruses showed that all belonged to clade 6B.1. 41 A(H1N1)pdm09 viruses were antigenically characterized, and all were antigenically similar (analyzed using HI with ferret antisera) to the reference 6B.1 virus A/Michigan/45/2015, representing the recommended influenza A(H1N1)pdm09 reference virus for the 201718 Northern Hemisphere influenza vaccines.
A (H3N2): Phylogenetic analysis of the HA genes from 256 A(H3N2) viruses revealed extensive genetic diversity with multiple clades/subclades co-circulating. The HA genes of circulating viruses belonged to clade 3C.2a (n=198), subclade 3C.2a1 (n=56) or clade 3C.3a (n=2). 68 influenza A(H3N2) viruses were antigenically characterized, and 67 (98.5%) A(H3N2) viruses tested were well-inhibited (reacting at titers that were within fourfold of the homologous virus titer) by ferret antisera raised against A/Michigan/15/2014 (3C.2a), a cell propagated A/Hong Kong/4801/2014-like reference virus representing the A(H3N2) component of 201718 Northern Hemisphere influenza vaccines.
B/Victoria: Phylogenetic analysis of five B/Victoria-lineage viruses indicate that all HA genes belonged to genetic clade V1A, the same genetic clade as the vaccine reference virus, B/Brisbane/60/2008. However, a small number of viruses identified in 2017 had a 6-nucleotide deletion (encoding amino acids 162 and 163) in the HA (abbreviated as V1A-2Del). One (50%) of two B/Victoria lineage viruses were well-inhibited by ferret antisera raised against cell -propagated B/Brisbane/60/2008 reference virus, representing a recommended B virus component of 201718 Northern Hemisphere influenza vaccines. One B/Victoria lineage virus reacted poorly (at titers that were 8-fold or greater reduced compared with the homologous virus titer) with ferret antisera raised against cell-propagated B/Brisbane/60/2008, and this virus had the two amino acid deletion in the HA of the V1A-2Del viruses.
B/Yamagata: Phylogenetic analysis of 81 influenza B/Yamagata-lineage viruses indicate that the HA genes belonged to clade Y3. A total of 57 influenza B/Yamagata-lineage viruses were antigenically characterized, and all were antigenically similar to cell propagated B/Phuket/3073/2013, the reference vaccine virus representing the influenza B/Yamagata-lineage component of the 201718 Northern Hemisphere quadrivalent vaccines.