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 47/2021, 925 of 66 557 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; 907 (98%) were type A and 18 (2%) were type B. Of 562 subtyped A viruses, 13 (2%) were A(H1)pdm09 and 549 (98%) A(H3). No type B viruses were ascribed to a lineage. 

For the season to date, more influenza type A (n=2 371, 92%) than type B (n=197, 8%) viruses have been detected. Of 1 564 subtyped A viruses, 63 (4%) were A(H1)pdm09 and 1 501 (96%) were A(H3). Of 2 influenza type B viruses ascribed to a lineage both were B/Victoria (99% of type B viruses were reported without a lineage).  

Genetic characterization

Up to week 47/2021, 161 A(H3) viruses have been characterized genetically, all of which belonged to clade 3C.2a1b.2a2. One A(H1)pdm09 virus characterized genetically for week 42 was not attributed to any clade and one A(H1)pdm09 virus for week 45 belonged to clade 6B.1A.5a.1. Up to week 47/2021, 2 B/Victoria viruses have been characterized genetically, one belonged to clade V1A.3a.2 and one to clade V1A.3.


ECDC published the October virus characterisation report that describes the available data from circulating viruses collected after 31 August 2020. This and previously published influenza virus characterization reports are available on the ECDC website.

Antiviral susceptibility of seasonal influenza viruses

Up to week 47/2021, 122 A(H3) viruses were assessed for susceptibility to neuraminidase inhibitors and 69 A(H3) viruses were assessed for susceptibility to Baloxavir marboxil. No amino acid substitutions previously associated with reduced susceptibility were identified.