NORWAY
Interim influenza virological and epidemiological season report prepared for the WHO Consultation on the Composition of Influenza Virus Vaccines for the Northern Hemisphere 2024/2025
Report
|Published
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.
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.
Following the A(H1N1) dominated 2022-2023 season, there was an increase in seroprevalence against A/Victoria/2570/2019 of the 5a.2 clade, although immunity was lower against the 5a.2a.1 clade. 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.
The current season is also A(H1N1) dominated however the early clear dominance has been challenged in recent weeks by an increase in A(H3N2) prevalence. Influenza B viruses have been much less common, all that have been typed are B/Victoria-lineage. Like the preceding season the timing of the current influenza season also shows an early peak during the Christmas period with a following decline, currently the trend is stable but elevated and we expect either a second peak or a long tail season. The Christmas peak was of clearly lower magnitude than the one of 2022-2023.
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 has been stable through weeks 2-4 2024.
The numbers of hospitalisations and ICU admissions with influenza began to increase around week 44/2023, reaching a peak in week 52/2023. As of week 4/2024, 2414 hospital admissions and 85 ICU admissions have been reported, which is significantly less than in the same time period in the preceding season 2022-2023.
15% (234/1523) of all influenza positive samples received for surveillance have been whole genome sequenced. A(H1N1) viruses predominated early in the season with the clade 5a.2a represented by A/Sydney/5/2021 and 5a.2a.1 represented by A/Wisconsin/67/2022 and A/Victoria/4897/2022. The genetic makeup is balanced between genAH1/Sydney/5/2021 and genAH1/Victoria/4897/2022 with no clear dominance to be observed. Within the clade 5a.2s the genAH1/Sydney/5/2021 show a wide genetic variability with several clusters evolving with their unique substitutions which continue to grow in the mid-season. Within the genAH1/Victoria/4897/2022 like viruses 3 defined clusters are observed with the biggest one carrying an S85P and R113K substitutions. A(H3N2) viruses have mostly been in the 2a.3a.1 subclade (A/Thailand/8/2022 group), with a couple of subgroups of which one with additional substitutions N122D and K276E is largest and expanding. B/Victoria strains are rarely detected in the early and mid-season of 2023/24, with the majority falling in the B/Catalonia/2279261NS/2023 subgroup.
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 number of distributed doses decreased by 5 % compared to the 2022/23 season. 1.12 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 clade 2.3.4.4b continued to be detected in wild birds in Norway. During autumn 2023 there was one outbreak of H5N1 in a poultry backyard flock and no outbreaks in commercial poultry flocks. No human cases have been detected, and the risk of human infection has been assessed as very low.
