Virus characteristics

For reports based on sentinel surveillance systems this season, most influenza viruses detected were type B with those assigned to a lineage being mainly B/Yamagata viruses, while of the type A viruses subtyped most were influenza A(H1N1)pdm09 viruses. Details of the distribution of viruses detected in sentinel-source specimens can be found in the Primary care data section.

Conversely, the majority of detections from non-sentinel systems this season have been influenza type A viruses, and of those typed most were A(H3N2). However, week 1/2018 was the first in which influenza type B detections were in the majority for non-sentinel specimens. The B/Yamagata lineage has predominated among type B viruses, as seen in sentinel systems. Further details are given in the section below.

To date, a higher proportion of influenza type A detections in non-sentinel specimens has been observed compared to sentinel source specimens. This may be related to the higher proportion of non-sentinel specimens being derived from hospital-based settings, with type B virus infections being generally milder and leading to less hospitalization than type A virus infections. Overall, A(H3N2) viruses often cause more severe disease in the elderly and A(H1N1)pdm09 in middle-aged patients. The proportions of influenza virus subtypes may vary between countries, possibly related to the contributions of sentinel and non-sentinel surveillance, which may lead to differences in (sub)type proportions within the Region.

Viruses detected in non-sentinel source specimens 

For week 1/2018, 10 615 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, 44% were type A and 56% type B viruses; the first time this season that in a week more type B viruses than type A viruses were detected in non-sentinel specimens. 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 58% were A(H3N2). Of influenza type B viruses ascribed to a lineage, 98% were B/Yamagata lineage and 2% were B/Victoria lineage.

Genetic characterization

For specimens collected since week 40/2017, genetic characterization of 298 viruses has been reported. Among 137 influenza A(H3N2) viruses, 88 (64%) fell in the vaccine virus component clade (3C.2a), and 49 (36%) 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. 1 A(H1N1)pdm09, 1 A(H3N2) and 3 B/Yamagata viruses were not attributed to any clade. For more information on virus characterizations for EU/EEA countries, see the WHO CC London November 2017 report.

Table. Viruses attributed to genetic groups, cumulative for weeks 40–1/2018

Phylogenetic group

Number of viruses

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


A(H1N1)pdm09 not attributable to any clade


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


A(H3N2) A/Singapore/INFIMH-16-0019/2016 (clade 3C.2a1) c


A(H3N2) not attributable to any clade


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


B/Norway/2409/2017 (Victoria lineage clade 1A Δ162-163)e


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


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 Deletion of K162 and N163 in the HA1 subunit of the hemagglutinin and antigenically different from the vaccine component.

f Vaccine component of quadrivalent vaccines for use in 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 124 viruses (60 A(H3N2), 29 A(H1N1)pdm09 and 35 type B) with collection dates since week 40/2017. One A(H3N2) virus showed evidence of reduced inhibition by neuraminidase inhibitors oseltamivir and zanamivir.