Influenza Virological and Epidemiological 2023-2024 season report

Key message

• The population immunity towards influenza prior to the 2023-24 season was assessed in a panel of residual sera collected in August 2023. Following the A(H1N1) dominated 2022-23 season, there was an increase in seroprevalence against A/Victoria/2570/2019(H1N1) of the pdm09 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 had 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 incidence of laboratory confirmed influenza rose rapidly, like in the preceding season, 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 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. 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.  
• Out of the circulating viruses, type A viruses predominated during the main outbreak period, but declined faster than type B during late winter and spring, leaving type B as the majority virus during weeks 17-22. Influenza A(H1N1) and A(H3N2) cocirculated, with H1N1 in majority overall and main driver of the winter outbreak with a peak in percent positives at 16 % in week 52. Low compared to many previous seasons. The activity stayed elevated for the following 9 weeks before it rapidly declined after week 10.  A large proportion (43 %) of the less commonly detected type B viruses was lineage typed in the National Influenza Centre, FHI, with 1021 Victoria-lineage and zero Yamagata-lineage detections. Influenza A were again in majority among the sporadic detections, with predominance of subtype H3N2 through July and August 2024.
• Severe outcomes: 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. Eleven influenza outbreaks were reported, of which ten from health care institutions and one from an immigration detention center.  
• Genetic characterization with whole genome sequencing of 17% of all influenza positive samples received for surveillance revealed that A(H1N1) viruses A/Norway/25089/2022 6B.1A.5a.2a.1 clade dominated in early season, but by mid-season viruses in the A/Sydney/5/2021 6B.1A.5a.2 clade were in slight majority. The circulating H3N2 viruses are categorized as belonging to the A/Thailand/8/2022 3C.2a1b.2a.2a.3a.1 group. The main subclade detected were J.2 with several genetically distinct clusters emerging late in the season, one with clear antigenic drift properties.  All influenza B viruses sequenced were B/Victoria lineage, belonging to the V1A.3a.2 clade. The viruses circulating were in well accordance with the composition of the influenza vaccine this season.  
• Antiviral resistance mutations and reduced susceptibility towards oseltamivir was observed for one influenza B virus this season. A new globally emerging double mutant H1N1 inducing resistance towards oseltamivir was also detected in one single case in Norway.
• Vaccination coverage among risk groups 18-64 years decreased compared to the 2022-2023 season and has decreased by 5 percentage points from the 2021-2022 season. The coverage rate for individuals above 65 years was 65 %, which is an increase of over 2 percentage points compared to last season. The total number of distributed doses decreased by 5 % compared to the 2022-2023 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 with 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.