Influenza virus characteristics, week 40 2024 to week 33 2025, EU/EEA

Detections

Within the reporting period, 354 455 influenza virus detections (sentinel and non-sentinel combined) were reported, of which 73% (257 761) were type A and 26% (90 873) were type B virus; another 1.6% (5 821) were reported untyped.

Of the 52 442subtyped influenza A viruses (20% of detected type A viruses), 31 638 (60%) were subtype A(H1)pdm09 and 20 804 (40%) were subtype A(H3). Of the 90 873 reported influenza type B viruses, the lineage for 9 279 (10%) was determined, with all except one virus falling into the B/Victoria/2/87 lineage. One
report on viral RNA belonging to the B/Yamagata/16/88 lineage was submitted. Attempts to culture or sequence this virus were unsuccessful due to a very low viral load.

A(H1N1)pdm09

For A(H1N1)pdm09 viruses, 17 countries reported 5 572 haemagglutinin (HA) sequences. All of the 5 560 sequences that passed quality filters fell in the clade 5a.2a (C.1) branch. A proportion of 16% were further characterised into clade 5a.2a.1 (D), represented by the 2023–25 northern hemisphere (NH) vaccine strain for egg-based vaccines, A/Victoria/4897/2022. Most (69%) of the 5a.2a.1 viruses carried T120A in comparison with A/Wisconsin/67/2022_Cell and belonged to subclade D.3, which is not represented by a reference strain; 19% belonged to subclade D.2, defined by the additional R113K substitution.

Among the 821 characterised A(H1)pdm09 viruses, the main reported categories were A/Sydney/5/2021-like, clade 5a.2a (C.1) and A/Victoria/4897/2022-like, clade 5a.2a.1 (D), representing 51% and 46%, respectively. 

In total, 5 293 A(H1N1)pdm09 viruses were assessed for antiviral susceptibility to oseltamivir and/or zanamivir and/or baloxavir marboxil. Reduced and highly reduced inhibition by oseltamivir was detected in two (0.6%) and four (1.3%) of 317 phenotypically tested viruses. By genotypic testing, reduced and highly reduced inhibition by oseltamivir were observed for four (<0.1%) and 25 (0.5%) viruses, respectively, of the remaining 4 690 tested viruses. One of 4 714
viruses (<0.1%) showed genotypic reduced inhibition to zanamivir. Two (0.1%) of 3 048 assessed A(H1N1)pdm09 viruses carried genetic markers associated with reduced susceptibility to baloxavir marboxil.

A(H3N2)

For A(H3N2) viruses, 17 countries reported 4 279 HA sequences, of which 4 277 passed quality criteria and were analysed phylogenetically. All A(H3) HA sequences fell into the branch of clade 2a.3a defined by E50K compared with 2a.3 representative virus A/Norway/24873/2021; out of these, the majority (>99%) fell into clade 2a.3a.1 (J), represented by NH 2024–2025 egg-based vaccine strain A/Thailand/8/2022_Egg. Fifteen viruses belonged to 2a.3b and fell into subclade G.1.3.1. Within clade 2a.3a.1, the SH 2025 vaccine strain A/Croatia/10136RV/2023 represents subclade J.2, defined by N122D and K276E compared with NH 2024–2025 vaccine strain A/Massachusetts/18/2022_Cell. Most viruses (78%) fell into this subclade, 57% of which fell on a branch with the N8D amino-acid substitution that does not have a reference representative or subclade designation. 

Among the 486 characterised A(H3) viruses, the main reported categories were A/Croatia/10136RV/2023-like, clade 2a.3a.1 (J.2) and A/Thailand/8/2022-like, clade 2a.3a.1 (J), representing 50% and 44%, respectively. In total, 4 267 A(H3N2) viruses were assessed for antiviral susceptibility to oseltamivir and/or zanamivir and/or baloxavir marboxil. Reduced inhibition by oseltamivir was detected in four (1.3%) of 298 phenotypically tested viruses, and reduced inhibition by zanamivir was detected in one (<0.5%) of the 292 phenotypically tested viruses. By genotypic testing, reduced and highly reduced inhibition by oseltamivir was observed for 12 (0.3%) and three (0.1%) of the 3 930 tested viruses, respectively, and reduced inhibition by zanamivir was observed for 10 (0.3%) of the 3 936 tested viruses. Three (0.1%) of 2 593 assessed A(H3N2) viruses carried genetic markers associated with reduced susceptibility to baloxavir marboxil.

B/Victoria

For B/Victoria viruses, 17 countries reported 4 292 HA sequences. Excluding 17 sequences that did not pass quality control, all B/Victoria sequences carried HA genes that fell into genetic clade V1A.3a.2 (C), represented by NH 2022– 2025 vaccine strain B/Austria/1359417/2021_Egg. The majority (58%) of viruses fell in the C.5.1 subclade, followed by 22% in C.5.7 and 19% in C.5.6. Additional minor proportions below 1% also populated subclade C.3 and C.5. For the subclades present in the reported data set, each contained a reference strain. Overall, B/Victoria evolution was characterised by multiple branches absent from defining amino-acid substitutions.

Among the 684 characterised B(Victoria) viruses, the main reported category was B/Austria/1359417/2021-like, clade V1A.3a.2 (C), representing 92%. In total, 4 063 B(Victoria) viruses were assessed for antiviral susceptibility to oseltamivir and/or zanamivir and/or baloxavir marboxil. Reduced inhibition by oseltamivir was detected in eight (3.6%) of 223 phenotypically tested viruses, and reduced inhibition by zanamivir was detected in six (2.8%) of the 215 phenotypically tested viruses. By genotypic testing, reduced inhibition by oseltamivir were observed for six (0.2%) of 3 473 assessed viruses. Genotypically reduced and highly reduced inhibition to zanamivir was reported in four (0.1%) and one (<0.1%) of 3 480 tested viruses (0.1%). One (<0.1%) of 2 699 assessed B/Victoria viruses carried genetic markers associated with reduced susceptibility to baloxavir marboxil.