NORWAY
Influenza Virological and Epidemiological season report prepared for the WHO Consultation on the Composition of Influenza Virus Vaccines for the Southern Hemisphere 2025
Report
|Published
During the 2023-24 season, both A(H1N1) 5a.2a and 5a.2a1 viruses circulated, with a slightly higher prevalence of the 5a.2a1 during the winter outbreak. 5a.2a.1 also caused the winter outbreak in 2021-22, however, the subclades were different. C.1.9 subclade have been dominating the latest months of this season. The A(H3N2) viruses have been mainly from the 2a.3a.1 clade, with J.1 during the winter and J.2 the latest months.
Key message
- The preceding 2022-2023-influenza outbreak developed early, with a sharp peak during Christmas/New Year. This peak was dominated by influenza A(H1N1) viruses with subclades 5a.2a and 5a.2a.1 cocirculating. There were two smaller subsequent peaks; in week 6 consisting of a mixture of A(H1N1), A(H3N2), and B/Victoria; and in week 12 consisting of B/Victoria-lineage viruses.
- Inter-season population immunity was assessed in a panel of residual sera collected in August 2023. Following the A(H1N1) dominated 2022-2023 season, there was an increase in seroprevalence against A/Victoria/2570/2019 of the 5a.2 clade, but with less immunity against the subclade 5a.2a.1. The increased seroprevalence was particularly prominent in the 0-4 years age group, where we in 2022 observed an immunity gap following the absence of influenza during the COVID-19 pandemic. Seroprevalence against A(H3N2) also increased or remained stable, and there was no sign of immune evasion with the new A/Thailand/8/2022 strain. Seroprevalence against B/Victoria-lineage virus increased from very low levels in 2022, likely reflecting the spread of influenza B during the latter part of the 2022-2023 season.
- In the 2023-2024 season covered in the present report, type A viruses predominated during the main outbreak period but declined faster than type B during late winter and spring, so that type B was in majority during weeks 17-22. A(H1N1) and A(H3N2) cocirculated, with H1N1 in majority overall. Among the much less common type B viruses, a large proportion (43 %) was lineage typed in the National Influenza Centre, with 1021 Victoria-lineage and zero Yamagata-lineage detections.
- Like in the preceding season the incidence of laboratory confirmed influenza rose rapidly toward an early peak during the Christmas/New Year period, albeit at lower intensity than the 2022-23 season. Detections declined only slightly after the New Year peak and was sustained at moderate level until falling under 10 % positivity rate in week 10/2024. By week 20 the rate had declined to 2 % and it has stayed below this through summer when influenza A again were in majority among the sporadic detections, with predominance of subtype H3N2 through July and August.
- The proportion of influenza-like illness (ILI) consultations in primary health care began to increase gradually from week 44/2023 and the epidemic threshold was crossed in week 49. Influenza activity peaked in week 52 when 1,4 % of the consultations were due to ILI, at low intensity level. The activity declined after week 52 and was stable on a low intensity level until it crossed below the epidemic threshold in week 9/2024.
- Between week 40/2023 and week 20/2024, a total of 4403 hospital admissions and 179 ICU admissions were reported, which is less than in the same period in the preceding season 2022-2023.
- 17% (720/4120) of all influenza positive samples received for surveillance have been whole genome sequenced. Among the A(H1N1) viruses A/Norway/25089/2022 6B.1A.5a.2a.1 clade dominated in early season but by mid-season diverse viruses in the A/Sydney/5/2021 6B.1A.5a.2 mother clade were in slight majority. During the late season three clusters were seen to be growing, one of which carries the N38D substitution. The circulating H3N2 viruses are categorized as belonging to the A/Thailand/8/2022 3C.2a1b.2a.2a.3a.1 group. The main subclades detected were J.2 (175/238) and J1 (50/238) with several genetically distinct clusters emerging late in the season. All influenza B viruses sequenced were B/Victoria lineage, belonging to the V1A.3a.2 clade. While all carried the D197E substitution some lacked the E183K (17/174) substitution defining B/Catalonia/2279261NS/2023-like virus. Recently two clusters emerged both carrying the D129N mutation.
- Vaccination coverage among risk groups younger than 65 years decreased compared to the 2022-2023 season. The coverage rate for individuals above 65 years was 64 %, which is at the same level as last season. The total number of distributed doses decreased by 5 % compared to the 2022/23 season. 1.13 million doses intended for use in risk groups and health care workers were distributed.
- Highly pathogenic avian influenza viruses (HPAIVs) H5N1 and H5N5 belonging to HA clade 2.3.4.4b continued to be detected in wild birds in Norway, albeit in far lower numbers compared to the 2023 summer and fewer outbreaks. During autumn 2023 there was one outbreak of H5N1 in a poultry backyard flock and there was one outbreak in a commercial poultry flock in February 2024. In the same month H5N5 was detected in two red foxes. No human cases have been detected, and the risk of human infection has been assessed as very low.
