... well, maybe not the first time. Now I remember the A/Ck/Shanxi/2/2006(H5N1)
and another strain (Egypt ?) with an unusually high mutation rate in HA in response to
the vaccine. (or not ? see below)
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https://www.sciencedirect.com/science/article/pii/S0042682209001810
clade 7
extensive genetic divergence at both the nucleotide and amino acid
https://journals.asm.org/doi/full/10.1128/JVI.00413-10
antigenically drifted H5N1 viruses, represented by A/chicken/Shanxi/2/06 (CK/SX/2/06), were
detected in chickens from several provinces in northern China.
The CK/SX/2/06-like viruses are reassortants with newly emerged HA, NA, and PB1 genes that
could not be protected against by the GS/GD/1/96-based vaccines.
CK/SX/2/06-like viruses caused mild disease in mice and could not replicate in ducks.
We documented the emergence of the latest variant of H5N1 (A/chicken/Shanxi/2/06 [CK/SX/2/06])
that broke through existing poultry vaccines. We show that this variant is less pathogenic in mice
and ducks than the earlier strains and propose that the variant was not selected by the use of
vaccines.