Virus characteristics

Details of the distribution of viruses detected in sentinel-source specimens can be found in the Primary care data section.

Viruses detected in non-sentinel source specimens 

For weeks 21–35/2018, 488 specimens from non-sentinel sources (such as hospitals, schools, primary care facilities not involved in sentinel surveillance, or nursing homes and other institutions) tested positive for influenza viruses, which was an increase of 68 compared to weeks 21–30/2018. Of the 488, 76% were type A and 24% type B viruses. Of the influenza A viruses that were subtyped, 54% were A(H3N2). The majority of influenza B viruses from non-sentinel specimens were not tested for lineage; 12 were tested, 10 of which were found to be B/Yamagata.

Genetic characterization

For specimens collected since week 21/2018, genetic characterization of 35 viruses has been reported (Table 3). All 24 influenza A(H1N1)pdm09 viruses fell in the A/Michigan/45/2015 vaccine component clade (6B.1), 1 A(H3N2) virus was reported as clade 3C.2a and 3 A(H3N2) viruses as subclade 3C.2a1 (with viruses defined by N171K, often with N121K, amino acid substitutions in the haemagglutinin). All 7 influenza B viruses belonged to clade 3 of the B/Yamagata lineage (represented by B/Phuket/3073/2013).

For more information on virus characterizations for EU/EEA countries, see the latest WHO CC London Influenza virus characterisation reports).

Table. Viruses attributed to genetic groups, cumulative for weeks 21/2018–30/2018

Phylogenetic group

Number of viruses

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


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


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


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


aVaccine component of vaccines for northern (2017–2018 and 2018-2019 seasons) and southern (2018 season) hemispheres

bVaccine component for southern hemisphere 2018 and northern hemisphere 2018-2019 seasons

cVaccine component of quadrivalent vaccines for use in northern hemisphere 2017–2018 and 2018-2019 seasons

dVaccine component for southern hemisphere 2018


The recommended composition of trivalent influenza vaccines for the 2017–2018 season in the northern hemisphere included 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 also recommended.

On 21 February 2018, WHO published influenza vaccine recommendations for the 2018–2019 season in the northern hemisphere. Two changes were recommended compared to the current trivalent and quadrivalent vaccine formulations used in the 2017–2018 season in the northern hemisphere. Similar to the recommended composition for the 2018 southern hemisphere vaccine, the A(H3N2) component was changed to an A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus. In trivalent vaccines the B component was switched to a B/Colorado/06/2017-like virus, representing the emergent strain of B/Victoria-lineage viruses with deletion K162 and N163 in the HA1 subunit. The A(H1N1)pdm09 component in trivalent and quadrivalent vaccines and the B/Yamagata component in quadrivalent vaccines remained the same.

Antiviral susceptibility testing

Neuraminidase (NA) inhibitor susceptibility has been assessed for 3 703 viruses with collection dates from week 40/2017: 1 539 type B, 990 A(H3N2), and 1 174 A(H1N1)pdm09. Two type B viruses carried the NA amino acid substitution D197N associated with reduced inhibition (RI) by oseltamivir and zanamivir, and 2 type B viruses showed RI by oseltamivir only. Nineteen A(H1N1)pdm09 viruses carried the NA amino acid substitution H275Y associated with highly reduced inhibition (HRI) by oseltamivir and 2 showed RI by zanamivir only. Two A(H3N2) viruses carried NA amino acid substitution R292K associated with HRI by oseltamivir and RI by zanamivir.