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 18/2022, 2 599 of 46 828 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 an influenza virus; 2 557 (98%) were type A and 42 (2%) were type B. Of 415 subtyped A viruses, 399 (96%) were A(H3) and 16 (4%) were A(H1)pdm09. Of 3 type B viruses ascribed to a lineage, all were Victoria lineage.


For the season to date, more influenza type A (n=119 111, 98%) than type B (n=2 065, 2%) viruses have been detected. Of 27 945 subtyped A viruses, 25 584 (92%) were A(H3) and 2 361 (8%) were A(H1)pdm09. Of 77 influenza type B viruses ascribed to a lineage, 96% were B/Victoria and 4% were B/Yamagata (96% of type B viruses were reported without a lineage).

Genetic characterization

Of the 251 genetically characterized A(H1)pdm09 viruses up to week 18/2022, the majority (227; 90%) belonged to clade 6B.1A.5a.1, represented by A/Guangdong-Maonan/SWL1536/2019. Only a few viruses belonged to clade 6B.1A.5a.2: 7 (3%) were represented by A/India/Pun-NIV312851/2021 and 16 (6%) were represented by A/Victoria/2570/2019, the virus component for the 2021/22 and 2022/23 northern hemisphere vaccines. 1 virus was not attributed to a clade.


Among the A(H3) viruses characterized up to week 18/2022, 2 737 were attributed to a clade. The majority 2 727 (>99%) belonged to clade 3C.2a1b.2a.2, represented by the A/Darwin/9/2021 component of 2022/23 northern hemisphere vaccines. Only 9 (<1%) were clade 3C.2a1b.1a viruses and 1 (<1%) virus fell into clade 3C.2a1b.2a.1.


Up to week 18/2022, 48 B/Victoria viruses were characterized. 26 of the viruses belonged to clade V1A.3a.2, represented by B/Austria/1359417/2021, the recommended vaccine virus for the 2022/23 northern hemisphere influenza season. 20 of the viruses from recent weeks fell into clade V1A.3 , which recently emerged in the European area, represented by B/Washington/02/2019, the recommended vaccine virus strain for the 2021/22 northern hemisphere influenza season. 2 viruses were not attributed to a clade.


7 viruses were characterized as B/Yamagata with 4 being B/Phuket/3073/2013-like while 3 viruses were not attributed to a clade. However, the possibility that these 7 viruses were derived from live attenuated influenza vaccine (LAIV) could not be excluded.


ECDC published the March virus characterization report that describes the available data from circulating viruses this influenza season: currently type A influenza virus circulation is dominating over type B, due mainly to A(H3) viruses. Vaccination remains the best protective measure for prevention of influenza. However, based on post-infection ferret antisera data, the predominant A(H3N2) viruses in circulation are not well recognized by antisera raised against viruses genetically and antigenically similar to the vaccine virus, indicating antigenic diversity. Therefore, it is possible that the A(H3) vaccine component may induce less good recognition of the prevalent A(H3) viruses, although preliminary VE data indicates a still moderate level of protection against laboratory confirmed infection. Clinicians should therefore consider early antiviral treatment of at-risk groups with influenza infection, according to local guidance, to prevent severe outcomes.


This and previously published influenza virus characterization reports are available on the ECDC website.


Antiviral susceptibility of seasonal influenza viruses

Up to week 18/2022, 2 198 viruses were assessed for susceptibility to neuraminidase inhibitors (1 469 A(H3), 203 A(H1)pdm09 and 38 B viruses genotypically and 444 A(H3), 31 A(H1)pdm09 and 13 B viruses phenotypically), and 1 529 viruses were assessed for susceptibility to baloxavir marboxil (1 314 A(H3), 187 A(H1)pdm09 and 28 B viruses genotypically). Phenotypically, no viruses with reduced susceptibility were identified and genotypically 2 A(H3) viruses with potentially reduced susceptibility to baloxavir marboxil were identified.


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