Influenza Virological and Epidemiological season report prepared for the WHO Consultation on the Composition of Influenza Virus Vaccines for the Southern Hemisphere 2026, September 2025
Report
|Published
The report gives an overview from Norway over the influenza season 2024-2025, dominating influenza subtypes and vaccine coverage.
Key message
- Laboratory confirmed influenza started to rise from baseline in late November 2024, crossed the virological outbreak threshold in week 52, and peaked around week 8 with a positivity rate of 35.5 % which is on par with the peak of the 2017-2018 season. Since week 40/2024, 44,930 infections with influenza A and 15,514 with influenza B have been detected by Norwegian laboratories, out of > 520,000 patients tested.
- Based on the weekly total numbers of detected influenza A and B, and the weekly proportions of subtypes and lineages, we estimate that A(H1N1) constituted approximately 46 % of the influenza cases, followed by B/Victoria-lineage with 28 % and A(H3N2) with 27 %.
- In age profile analysis of detected cases, the 0–4-year-olds were more than twice more likely to be diagnosed with A(H1N1) than other ages, similar to profiles in previous seasons. Surprisingly, for A(H3N2) detections, the elderly were not over-represented as they have been in previous years. For influenza B/Victoria, younger age groups, particularly 5–14-year-olds, were much more likely to be diagnosed than those who are older; elderly were particularly under-represented.
- This far, 14,5 % (742/5,115) of all influenza positive samples received for surveillance have been whole genome sequenced. 101 viruses, representing the spectrum of genetic variants we have observed, have been shared with WHO. Among the A(H1N1) viruses, the 5a.2a clade was dominant in early season but was replaced by the 5a.2a.1 viruses from March 2025 onwards. Among the 5a.2a.1 viruses the D.3 subclade was the most common (27%) followed by D.3.1 (26%). A(H3N2) viruses predominantly (99 %) belonged to the 2a.3a.1 clade, of which most (58%) were subclade J.2, followed by J.2.2 (34%). In August, subclade J.2.4 viruses with some additional mutations have appeared. All sequenced influenza B viruses belonged to the B/Victoria V1A.3a.2 clade, with 56% being subclade C.5.1, 20% C.5.7, 15% C.5.6, 7% C.3, and 2% C.5.
- Seroepidemiological analysis of protective antibody responses against relevant strains of all influenza subtypes indicate that immunity was at a relatively high level in late summer 2024. HAI titres against tested influenza strains belonging to clades 5a.2a.1 of A(H1N1), 2a.3a.1 of A(H3N2), and 3A.2 of B/Victoria increased or remained stable for all age groups collected in 2024, compared to sera collected in 2023. The “immunity gap” seen in the youngest age group after the COVID-19 pandemic has now been closed.
- The proportion of influenza-like illness (ILI) began to rise gradually from week 50/2024 and the epidemic threshold was crossed in week 02/2025, two weeks later than crossing the outbreak threshold for per cent test positives. Influenza activity peaked in week 9 when 2,8 % of the consultations were due to influenza-like illness, which indicates a medium intensity level according to the MEM-thresholds. The ILI indicator resided at medium level for four weeks.
- Between week 40/2024 and week 34/2025, 10,657 (190.5 per 100,000 inhabitants) samples positive for influenza virus were reported among hospitalized patients, with the highest incidence among those aged 65-79 and 80+, followed by children under 5 years. Influenza A virus was most common, while influenza B virus to a larger extent affected children. Between week 40/2024 and week 20/2025, 278 (5.0 per 100,000 inhabitants) intensive care admissions with influenza were reported, and 375 influenza associated deaths were registered. The numbers of influenza hospital and intensive care admissions and influenza associated deaths were greater than the last previous seasons indicating that the 2024-2025 influenza season was more severe than the previous few seasons.
- The vaccine coverage for the age group 65 years and older was 66% per week 20/2025, and the total number of distributed doses in Norway was 1.56 million. This is at the same level as last season. 99 percent of the doses were administered before the epidemic threshold was reached.
- Highly pathogenic avian influenza viruses (HPAIV) belonging to H5 HA clade 2.3.4.4b, during most of the period typically subtype H5N5 but more recently a rise in H5N1, continued to be detected in wild birds in Norway. The detections were fewer and more scattered compared to the summer of 2023. H5N5 was last winter also detected in two euthanized carnivores, a lynx and an otter, and in a red fox carcass. In summer 2025, HPAIV H5N5 was detected in polar foxes on the archipelago of Svalbard. There was one outbreak of HPAIV AH5N5 in a poultry backyard flock in November 2024. No human cases have been detected, and the general risk for human infection is assessed as very low.