Virus characteristics


Viruses detected from non-sentinel sources 

 For weeks 21–26/2017, 1 029 specimens from non-sentinel sources (such as hospitals, schools, non-sentinel primary care facilities, nursing homes and other institutions) tested positive for influenza viruses (Table) and the number of detections showed a decreasing trend over this period. Of these detections, 12% were type A (81% of all subtyped viruses being A(H3N2)) and 88% type B (95% of those which have been ascribed to a lineage were Yamagata lineage).

Table. Influenza viruses detected in non-sentinel-source specimens, by virus (sub)type,  cumulatively for weeks 21-26/2017

Virus type and subtype

Weeks 21-26/2017



Influenza A









A not subtyped



Influenza B



B/Victoria lineage



B/Yamagata lineage



Unknown lineage



Total detections / Total tested

1 029 / 21 507


For influenza type percentage calculations, the denominator is total detections; for subtype and lineage, it is total influenza A subtyped and total influenza B lineage determined, respectively; as not all countries have a true non-sentinel testing denominator, no percentage calculations for total tested are shown.



Genetic characterization

For specimens collected between weeks 21/2017 and 26/2017, genetic characterization of 3 viruses has been reported. Two were B/Yamagata lineage viruses and one fell into the A(H3N2) 3C.2a1 subclade defined by N171K amino acid substitution, often with N121K, in the haemagglutinin. Viruses in this clade have been antigenically similar to the vaccine component clade (3C.2a), but both clades are evolving rapidly with the emergence of several virus clusters defined by additional amino acid substitutions in the haemagglutinin, thereby requiring continued monitoring of antigenic characteristics. See also the WHO CC London February 2017 report.

The recommended composition of trivalent influenza vaccines for the 2016–2017 season in the northern hemisphere was for inclusion of an A/California/7/2009 (H1N1)pdm09-like virus; an A/Hong Kong/4801/2014 (H3N2)-like virus; and a B/Brisbane/60/2008-like virus (B/Victoria lineage). For quadrivalent vaccines a B/Phuket/3073/2013-like virus (B/Yamagata lineage) was recommended. On 2 March 2017, WHO announced the recommended vaccine composition for the 2017–2018 season in the northern hemisphere. The recommendations matched those for the 2016–2017 season, but for the A(H1N1)pdm09 component being changed to an A/Michigan/48/2015-like virus (clade 6B.1).

Mid-season vaccine effectiveness (VE) estimates for all age groups against A(H3N2) illness from Canada (42%), from the US (43%) and from Europe (38%) were consistent with early estimates from Finland and Sweden.

Antiviral susceptibility testing

Neuraminidase inhibitor susceptibility has been assessed for 3 influenza viruses (1 A(H3N2) and 2 type B) with collection dates between week 21/2017 and 26/2017. None showed evidence of reduced inhibition to either oseltamivir or zanamivir.