Virus characteristics

For reports based on sentinel surveillance systems, most influenza viruses detected were type B viruses with those assigned to a lineage being mainly B/Yamagata viruses, while of the type A viruses subtyped A(H1N1)pdm09 have been in the majority. Details can be found here.

Conversely, most detections from non-sentinel systems have been influenza type A viruses, with A(H3N2) being the majority, while the B/Yamagata lineage has also predominated among type B viruses. Details below.

Viruses detected in non-sentinel source specimens 

For week 48/2017, 679 specimens from non-sentinel sources (such as hospitals, schools, primary care facilities not involved in sentinel surveillance, nursing homes and other institutions) tested positive for influenza viruses. Of these, 57% were type A and 43% type B viruses. The majority of viruses from non-sentinel specimens were not subtyped or assigned to a lineage.

While relatively few of the viruses detected in non-sentinel samples since week 40/2017 have been ascribed to a subtype or lineage, of all subtyped A viruses 82% were A(H3N2). Of influenza type B viruses ascribed to a lineage (n=76), 92% were B/Yamagata lineage and 8% were B/Victoria lineage.

Genetic characterization

For specimens collected since week 40/2017, genetic characterization of 122 viruses has been reported (Table 3). Among 74 influenza A(H3N2) viruses, 45 (61%) fell in the vaccine virus component clade (3C.2a), and 29 (39%) in subclade 3C.2a1 with viruses defined by N171K, often with N121K, amino acid substitutions in the haemagglutinin. Viruses in these 2 groups are antigenically similar, but both clade and subclade 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. Three B/Yamagata viruses were not attributed to any clade.

Table. Viruses attributed to genetic groups, cumulative for weeks 40–48/2017

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/Brisbane/60/2008 (Victoria lineage clade 1A)b,d


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


B/Yamagata lineage not attributed to any clade


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 Vaccine component for Southern Hemisphere 2018 season

d Vaccine component of quadrivalent vaccines for use in Southern Hemisphere 2018 season

e Vaccine component of quadrivalent vaccines for use in Northern 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 – 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

Neuraminidase inhibitor susceptibility has been assessed for 55 viruses (33 A(H3N2), 11 A(H1N1)pdm09 and 11 type B) with collection dates since week 40/2017. One A(H3N2) virus showed evidence of reduced inhibition by neuraminidase inhibitors oseltamivir and zanamivir.