Virus characteristics

 For detections from both sentinel and non-sentinel surveillance systems, most viruses subtyped or assigned to a lineage this season were identified as A(H3N2) and B/Yamagata viruses, respectively.

Details of the distribution of viruses detected in sentinel-source specimens can be found here.

Viruses detected from non-sentinel sources 

For week 45/2017, 216 specimens from non-sentinel sources (such as hospitals, schools, non-sentinel primary care facilities, nursing homes and other institutions) tested positive for influenza viruses. Of these, 74% were type A and 26% type B viruses. The majority of viruses from non-sentinel specimens were not subtyped or assigned to a lineage.

While few of the viruses detected in non-sentinel samples since week 40/2017 have been ascribed to a subtype or lineage, of all typed viruses, 73% were type A and 79% of those subtyped were A(H3N2). Of influenza type B viruses ascribed to a lineage (n=18), 89% were B/Yamagata lineage and 11% were B/Victoria lineage.

Genetic characterization

For specimens collected since week 40/2017, genetic characterization of 27 viruses has been reported. Among 21 influenza A(H3N2) viruses, 17 (81%) fall in the vaccine virus component clade (3C.2a), and 4 (19%) in subclade 3C.2a1 of clade 3C.2a viruses defined by N171K, often with N121K, amino acid substitutions in the haemagglutinin. Viruses in these two clades are antigenically similar, 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.

Table. Viruses attributed to genetic groups, cumulative for weeks 40–45/2017

Phylogenetic group

Number of viruses

A(H1N1)pdm09 A/Michigan/45/2015 (clade 6B.1)a


A(H3N2) A/Hong Kong/4801/2014 (clade 3C.2a)b


A(H3N2) A//Singapore/INFIMH-16-0019/2014 (clade 3C.2a1) c 


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


a Vaccine component of vaccines for both Northern (2017-2018 season) and Southern (2018 season) hemispheres

b Vaccine component for Northern Hemisphere 2017–2018 season

c Trivalent vaccine components for Southern Hemisphere 2018 season, not included in the trivalent vaccine for the Northern Hemisphere 2017-2018 season.


The recommended composition of trivalent influenza vaccines for the 2017–2018 season in the northern hemisphere includes an A/Michigan/45/2015 (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 28 September 2017, WHO recommended two changes, compared to the current trivalent vaccine recommended for the 2017–2018 northern hemisphere influenza season, in trivalent vaccine composition for the 2018 season in the southern hemisphere. The recommendations matched the A(H1N1)pdm09 component for the 2017–2018 northern hemisphere season, but the A(H3N2) component was changed to an A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus and the B component was switched to a B/Yamagata-lineage virus. These changes were made due to the emergence of numerous genetic subclades of A(H3N2) viruses - none of which showed significant antigenic drift compared to the vaccine component - and reports of low vaccine effectiveness against these viruses, while for type B viruses, the B/Yamagata lineage predominated by a large margin in the course of the 2017 southern hemisphere season. See also the ECDC commentary.


Antiviral susceptibility testing

No reports on antiviral susceptibility for viruses with collection dates in weeks 40–45/2017 have been received.