Virus characteristics


Viruses detected from non-sentinel sources 

For week 16/2017, 847 specimens from non-sentinel sources (such as hospitals, schools, non-sentinel primary care facilities, nursing homes and other institutions) tested positive for influenza viruses (Figure, Table).

Of these, 21% were type A (with 94% of the subtyped viruses being A(H3N2)), and 79% type B. The increase in proportion of type B viruses corresponds to the sentinel detection data, however the number of influenza B viruses detected remained relatively low and similar to that seen in recent weeks.

Whilst no subtype or lineage was determined for the majority of influenza viruses since week 40/2016, cumulative distributions of types and type A subtypes similar to those among sentinel detections have been observed: of all typed viruses, 89% were type A, with 99% of those subtyped being A(H3N2). Of 1 319 influenza type B viruses ascribed to a lineage, 76% were B/Yamagata lineage and 24% were B/Victoria lineage, which differs from sentinel detections where B/Victoria lineage and B/Yamagata lineage viruses have been more evenly distributed this season. The difference is mainly driven by the proportion of influenza B lineage detections in sentinel specimens in Latvia, Norway and Slovenia (B/Yamagata lineage predominant).

Table. Influenza viruses detected in non-sentinel-source specimens, by virus (sub)type, week 16/2017 and cumulatively

Virus type and subtype

Current Week

Season 2016-2017





Influenza A



109 502










39 152


A not subtyped



69 992


Influenza B



13 890


B/Victoria lineage





B/Yamagata lineage



 1 004


Unknown lineage



12 571


Total detections / Total tested

847 /8 173


123 392/557 804


aFor 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 since week 40/2016, genetic characterizations of 3 425 viruses have been reported (Table). Among 3 104 A(H3N2) viruses, 916 fell in the vaccine component clade (3C.2a) and 2 149 in the 3C.2a1 subclade defined by N171K amino acid substitution, and often with N121K, in the haemagglutinin. Viruses in these two clades have been antigenically similar, but both clades are evolving rapidly with 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.

 Table. Viruses attributed to genetic groups, cumulative for weeks 40/2016–16/2017

Phylogenetic group

Number of viruses

A(H1N1)pdm09 A/Michigan/45/2015 (subgroup 6B.1)b, c


A(H1N1)pdm09 A/South Africa/3626/2013 (subgroup 6B)


A(H3N2) A/Bolzano/7/2016 (subgroup 3C.2a1)


A(H3N2) A/Hong Kong/4801/2014 (subgroup 3C.2a)a, b, c


A(H3N2) A/Samara/73/2013 (subgroup 3C.3)


A(H3N2) A/Switzerland/9715293/2013 subgroup (3C.3a)


A(H3N2) A/Stockholm/28/2014 (subgroup3C.3b)


A(H3N2), subgroup not listed


B/Brisbane/60/2008 (Victoria lineage clade 1A)a, b, c


B/Phuket/3073/2013 (Yamagata lineage clade 3)d


a Vaccine component for Northern Hemisphere 2016–2017 season

b Vaccine component for Southern Hemisphere 2017 season

c Vaccine component for Northern Hemisphere 2017-2018 season

d Vaccine component of quadrivalent vaccines for use in both Northern and Southern Hemisphere


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).

Early monitoring of vaccine effectiveness (VE) in Finland and Sweden (Stockholm County) suggested levels of effectiveness in persons aged 65 years or older (32% and 28% VE, respectively) similar to estimates from annual multi-country studies covering the 2011–2012 and 2014–2015 seasons. More recent VE estimates for all age groups against A(H3N2) illness from Canada (42%), from the US (43%) and from Europe (38%) were consistent with the early estimates from Finland and Sweden.

Antiviral susceptibility testing

Neuraminidase inhibitor susceptibility has been assessed for 1 816 influenza viruses (1 662 A(H3N2), 29 A(H1N1)pdm09 and 125 type B) with collection dates since week 40/2016. One A(H3N2) virus, from a specimen collected in week 4/2017, showed reduced inhibition by oseltamivir in phenotypic assay. One A(H3N2) virus, from a specimen collected in week 01/2017, showed reduced inhibition by zanamivir in phenotypic assay.