Virus Characteristics

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

Non-sentinel virological data
  • For week 02/2021, 11 of 21 822 non-sentinel specimens (from sources such as hospitals, schools, primary care facilities not involved in sentinel surveillance, or nursing homes and other institutions) tested positive for an influenza virus: 2 were type A and 9 were type B.

 

  • Since the beginning of the season, 206 of 238 442 non-sentinel specimens tested positive for influenza viruses; 96 (46.6%) were type A and 110 (53.4%) type B. Of 42 subtyped A viruses, 10 (23.8%) were A(H1)pdm09 and 32 (76.2%) were A(H3). Of 110 type B viruses, only 6 were ascribed to a lineage: 5 B/Victoria and 1 B/Yamagata.
Genetic characterization
 

No virus characterization data for viruses detected in weeks 40/2020-02/2021 have been reported.

 

Note: It is essential that reporting laboratories submit any data they have generated to GISAID (and thereby TESSy) as soon as possible, together with sharing influenza-positive samples with WHO CC, London for more detailed characterization.

 

ECDC published the December virus characterisation report. No antigenic data relating to viruses detected in the course of the 2020-2021 influenza season had been generated and the report was based on an analysis of seasonal influenza HA sequenced most recently and submitted to GISAID. The following text is repeated from the Summary text of this report with minor modification. Previously published influenza virus characterization reports are also available on the ECDC website.

 

A(H1N1)pdm09 viruses

The vast majority of A(H1N1)pdm09 viruses had continued to fall in genetic subclade 6B.1A5, mostly in the 6B.1A5A group with few in the 6B.1A5B group. 6B.1A5A viruses have continued to evolve and two subgroups have emerged designated 6B.1A5A+187V/A, representatives of which are recommended for use in the northern hemisphere 2020-2021 season, and 6B.1A5A+156K, an antigenically distinct group representatives of which are recommended for use in the southern hemisphere 2021 season. Very few A(H1N1)pdm09 viruses have been detected worldwide in the course of the 2020-2021 season.

 

A(H3N2) viruses

Recently circulating A(H3N2) viruses had continued to fall in clades 3C.2a and 3C.3a, with the vast majority of clade 3C.2a viruses being in the 3C.2a1b group which has now been divided into four subgroups designated 3C.2a1b+T131K-A, 3C.2a1b+T131K-B, 3C.2a1b+T135K-A and 3C.2a1b+T135K-B. Antisera raised in ferrets show high levels of clade/group specificity, though there is some subgroup cross-reactivity. Viruses representative of subgroup 3C.2a1b+T135KB have been recommended for use in influenza vaccines for the northern hemisphere 2020-2021 and southern hemisphere 2021 seasons. To date, while low numbers have been reported, the great majority of A(H3N2) viruses from the 2020-2021 season have been detected in Asia, falling in subgroup 3C.2a1b+T131K-A and splitting into two clusters that each contain significant numbers of HA1 amino acid substitutions some of which are likely to alter antigenicity.

 

B/Victoria viruses

Of four antigenically distinct groups of viruses in the B/Victoria-lineage, only two had circulated recently, small numbers of that designated subclade 1A (Δ2) with a two amino acid deletion in HA1 and that designated subclade 1A(Δ3)B with a three amino acid deletion in HA1 being hugely dominant. Viruses representative of subclade 1A(Δ3)B have been recommended for use in influenza vaccines for the northern hemisphere 2020-2021 and southern hemisphere 2021 seasons. To date for the 2020-2021 season similar numbers to those for A(H3N2) have been detected with the great majority falling in a group defined by HA1 amino acid substitutions N150K, G184E, N197D/E (loss of a glycosylation site) and R279K, notably in some southern provinces of China.

 

B/Yamagata viruses

When the report published in December was written, all B/Yamagata viruses for which full-length HA sequences were available belonged to genetic clade 3 and contained at least two HA amino acid substitutions (HA1 L172Q and M251V) compared to B/Phuket/3073/2013-like viruses which have been recommended for use in quadrivalent influenza vaccines for the northern hemisphere 2020-2021 and southern hemisphere 2021 seasons. The antigenic effects of these amino acid substitutions have been minimal as assessed in earlier reports. To date, no viruses of this lineage have been detected in the course of the 2020-2021 season.

 

Antiviral susceptibility of seasonal influenza viruses

For week 02/2021 and since the beginning of the season, no influenza viruses were tested for susceptibility to neuraminidase inhibitors.