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 41/2021, 139 of 36 522 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; 83% were type A and 17% were type B. Of 76 A viruses subtyped, 93% were A(H3), and no B viruses were ascribed to a lineage  

For the season, 233 of 78 053 specimens tested positive for influenza viruses; 81% were type A and 19% were type B. Of the subtyped influenza A viruses 94% were A(H3). Croatia reported 23% positivity for non-sentinel specimens. 

Genetic characterization

Up to week 41/2021, four A(H3) viruses have been characterised genetically, all of which belonged to clade 3C.2a1b.2a2 and were reported to the category ‘AH3 attributed to recognised group in current guidance but not listed here’. 


ECDC published the September 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 41/2021, four A(H3) viruses were assessed for susceptibility to neuraminidase inhibitors and no amino acid substitutions previously associated with reduced susceptibility were identified.