Seasonal influenza circulation patterns and projections for Feb 2018 to Feb 2019

This report details current seasonal influenza circulation patterns as of Feb 2018 and makes projections up to Feb 2019 to coincide with selection of the 2018-2019 Northern Hemisphere vaccine strain. This is not meant as a comprehensive report, but is instead intended as particular observations that weve made that may be of relevance. Please also note that observed patterns reflect the GISAID database and may not be entirely representative of underlying dynamics. All analyses are based on the nextflu pipeline [1] with continual updates posted to nextflu.org.\n\nA/H3N2: H3N2 diversity has largely been replaced by subclades A1b, A2 and A3 within 3c2.A. Subclades A1b and A2 predominate in the population and each shows increases in frequency, mutations at epitope sites and evidence for minor changes to antigenic phenotype. Clade A1b may be marginally fitter than clade A2, but we expect both clades to persist into the future without a clear immediate winner.\n\nA/H1N1pdm: A clade comprising mutations S74R, S164T and I295V has recently swept to fixation. The rapidity of this sweep suggests a selective origin. However, there is no evidence of antigenic change.\n\nB/Vic: Very little B/Vic activity has been observed in recent months. A clade with a two codon deletion at sites HA1:162/163 has gradually risen in frequency. HI measurements suggest an 8 to 16-fold titer drop relative to the vaccine strain, but this antigenic change has not yet resulted in a rapid rise of this variant.\n\nB/Yam: Europe experienced a strong and early B/Yam season in absence of amino acid variation in HA or antigenic diversity. However, several mutations in NA have rapidly swept or risen to intermediate frequencies.