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 10/2023, 6 693 of 55 690 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; 4 035 (60%) were type B and 2 658 (40%) were type A. Of 362 subtyped A viruses, 272 (75%) were A(H1)pdm09 and 90 (25%) A(H3). Of 135 type B viruses ascribed to a lineage, all were B/Victoria.

For the season to date, more influenza type A (n=183 847, 81%) than type B (n=44 358, 19%) viruses have been detected. Of 53 954 subtyped A viruses, 29 581 (55%) were A(H1)pdm09 and 24 373 (45%) were A(H3). Of 2 985 influenza type B viruses ascribed to a lineage, all were B/Victoria (93% of type B viruses were reported without a lineage).

Genetic characterization

Of the 2 124 genetically characterized A(H1)pdm09 viruses up to week 10/2023, 1 142 were attributed to clade 6B.1A.5a.2, of which 596 (52%) were represented by A/Norway/25089/2022, 513 (45%) by A/Sydney/5/2021 and 33 (3%) by A/Victoria/2570/2019. Four (<1%) were attributed to clade 6B.1A.5a.1 represented by A/Guangdong-Maonan/SWL1536/2019. 978 (46%) viruses could not be attributed to a subgroup in the guidance.

Among the 2 234 A(H3) viruses characterized up to week 10/2023, 2 121 were attributed to clade 3C.2a1b.2a.2, of which 1 313 (62%) were represented by A/Bangladesh/4005/2020, 663 (31%) by A/Slovenia/8720/2022 and 145 (7%) by A/Darwin/9/2021. 110 (5%) viruses could not be attributed to a subgroup in the guidance. Only 3 viruses were ascribed to clade 3C.2a1b.1a represented by A/Denmark/3264/2019.

Up to week 10/2023, 627 B/Victoria viruses were characterized, 344 (55%) of which were attributed to clade V1A.3a.2 represented by B/Austria/1359417/2021. 283 (45%) viruses could not be attributed to a subgroup in the guidance.


Currently, WHO Europe and ECDC’s December virus characterization report is available and describes available data from circulating viruses for the early weeks of the 2022-2023 influenza season: type A influenza virus circulation dominated over type B, with similar proportions of circulating A(H3) and A(H1)pdm09 viruses. Vaccination remains the best protective measure for prevention of influenza. 


Previously published influenza virus characterization reports are available on the ECDC website and the WHO web site.



Antiviral susceptibility of seasonal influenza viruses

Up to week 10/2023, 3 491 viruses were assessed for susceptibility to neuraminidase inhibitors (1 265 A(H1)pdm09, 1 205 A(H3) and 507 B viruses genotypically and 269 A(H3), 177 A(H1)pdm09 and 68 B viruses phenotypically), and 2 657 viruses were assessed for susceptibility to baloxavir marboxil (1 500 A(H3), 701 A(H1)pdm09 and 456 B viruses genotypically). Genotypically, two (H1)pdm09 viruses were found to carry the NA H275Y marker, indicative of highly reduced inhibition (HRI) by oseltamivir and peramivir, and phenotypically no viruses with reduced susceptibility were identified. No markers of reduced susceptibility to baloxavir marboxil were detected.



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