Virus characteristics

Viruses detected from non-sentinel sources 

For week 11/2017, 2 175 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 and Table). Of these, 55% were type A (with 97% of the subtyped viruses being A(H3N2)), and 45% type B. The increase in proportion of type B viruses corresponds to the data seen in sentinel detections, however the number of B viruses detected remained lower than the number of type A viruses.

Fig. Influenza virus detections in non-sentinel-source specimens by type and subtype, by week

Whilst no subtype or lineage was determined for the majority of influenza viruses, similar cumulative distributions of types and type A subtypes as seen in sentinel detections have been observed since week 40/2016: of all typed viruses, 92% were type A, with 99% of those subtyped being A(H3N2). Of 993 influenza type B viruses ascribed to a lineage, 71% were B/Yamagata lineage and 29% were B/Victoria lineage (Table 2), which differs from sentinel detections where B/Victoria lineage and B/Yamagata lineage viruses have been 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).

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 11/2017 and cumulatively


Current Week

Season 2016-2017

Virus type and subtype





Influenza A

1 202


99 222










37 805


A not subtyped



61 093


Influenza B



9 192


B/Victoria lineage





B/Yamagata lineage





Unknown lineage



8 199


Total detections/

Total  tested

2 175 /15 430


108 414/496 078


 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 2 573 viruses have been reported (Table 3). Among 2 334 A(H3N2) viruses, 737 fell in the vaccine component clade (3C.2a), and 1 566 in the 3C.2a1 subclade defined by N171K, often with N121K, amino acid substitutions 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.

 Table. Viruses attributed to genetic groups, cumulative for weeks 40/2016–11/2017

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)

1 566

A(H3N2) A/Hong Kong/4801/2014 (subgroup3C.2a)a, b, c


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) virus 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, however with the A(H1N1)pdm09 component changed to an A/Michigan/48/2015-like virus (clade 6B.1).

Early monitoring of vaccine effectiveness (VE) in Finland and Stockholm county suggested levels of effectiveness in persons aged 65 years or older (32% and 28% vaccine effectiveness, respectively) similar to estimates from annual multicountry 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%) are consistent with the early estimates from Finland and Stockholm county.

Given typically suboptimal vaccination coverage and the partial effectiveness of influenza vaccines, rapid use of neuraminidase inhibitors (NAIs) for laboratory-confirmed or probable cases of influenza infection should be considered for vaccinated and non-vaccinated patients at risk of developing complications.

Antiviral susceptibility testing

Neuraminidase inhibitor susceptibility has been assessed for 1 226 viruses (1 122 A(H3N2), 22 A(H1N1)pdm09 and 82 type B) with collection dates since week 40/2016. One A(H3N2), from week 2/2017, showed reduced inhibition to oseltamivir in phenotypic assay.