Interim Influenza Virological and Epidemiological season report prepared for the WHO Consultation on the Composition of Influenza Virus Vaccines for the Northern Hemisphere 2025/2026

Key message

• Laboratory confirmed influenza started to rise from baseline in late November 2024 and crossed the virological outbreak threshold in week 52. Further growth resumed in week 3/2024 and by week 5 reached a positivity rate of 26.7 % which is the highest since the peak week during the 2017/2018 season. Since week 40/2024, 16,430 infections with influenza A and 4,552 with influenza B have been detected by Norwegian laboratories.  
• Approximately 80% are influenza A, out of which approximately 60 % are subtype H1N1 and 40% H3N2. All 629 investigated influenza B viruses belonged to the Victoria lineage.  
• 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. This age group was also twice as likely to have an A(H3N2) detection. Surprisingly, the elderly were not over-represented among A(H3N2) cases, as they have been in previous years. For influenza B/Victoria, younger age groups, particularly 5–14-year-olds, were much more likely than older persons to be diagnosed, while the elderly were under-represented.  
• This far, 9,4 % (216/2324) of all influenza positive samples received for surveillance have been whole genome sequenced. 81 viruses, representing all significant genetic variants we have seen, have been shared with WHO. Among the A(H1N1) viruses, the 5a.2a clade has been predominant (94 %), with very limited circulation of the 5a.2a.1 clade. 5a.2a clade viruses are spread across several subclades, with C.1.9.3 being most common. A(H3N2) viruses predominantly (97 %) belong to the 2a.3a.1 clade, of which half are subclade J.2.2, followed by J.2 (38 %) and J.2.1 (6 %). All sequenced influenza B viruses have belonged to the B/Victoria V1A.3a.2 clade, with 39 % being subclade C.5.1, 29 % C.3, 16 % C.5.6, and 13 % C.5.7.  
• 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. Presently, influenza activity is still increasing and in week 5 1,6 % of the consultations in primary health care were due to influenza-like illness, which is still at low intensity level according to the ILI MEM-thresholds.  
• Between 2024-w40 and 2025-w05, 2,582 (46.5 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.  In the same period, 78 (1.4 per 100,000 inhabitants) intensive care admissions with influenza were reported.  
• The vaccine coverage for the age group 65 years and older was 65% per week 5/2025, and the total number of distributed doses in Norway was 1.57 million. This is at the same level as last season. 99 % of the doses were administered before the epidemic threshold was passed. 
• Highly pathogenic avian influenza viruses (HPAIV) belonging to H5 HA clade 2.3.4.4b and recently mostly subtype H5N5, continued to be detected in wild birds in Norway, but in much lower numbers compared to the summer of 2023 and with fewer outbreaks. Infection with H5N5 has also recently been detected in two euthanized carnivores, a lynx and an otter. No human cases have been detected, and the general risk for human infection is assessed as very low.