Interim Influenza Virological and Epidemiological season report prepared for the WHO Consultation on the Composition of Influenza Virus Vaccines for the Northern Hemisphere 2023/2024
The preceding 2021-2022-influenza season developed unusually late, only after the distancing measures against COVID-19 were lifted in February, and at a time when the Omicron-variant driven main pandemic wave was on its decline. The influenza outbreak peaked around week 15, was of low-to medium magnitude, and influenza A(H3N2) viruses predominated.
- The preceding 2021-2022-influenza season developed unusually late, only after the distancing measures against COVID-19 were lifted in February, and at a time when the Omicron-variant driven main pandemic wave was on its decline. The influenza outbreak peaked around week 15, was of low-to medium magnitude, and influenza A(H3N2) viruses predominated.
- Seroprevalence against A/Victoria/2570/2019(H1N1) was at a moderate level in sera collected in August 2022. However, there was significantly less antibodies against the A/Norway/25089/2022 strain, which has been a prominent subvariant during the H1 outbreak seen in early winter. Seroprevalence against recent B/Victoria-lineage virus was at a low level, especially in the younger age groups, suggesting a high degree of susceptibility.
- The current season started early with outbreak threshold of 10 % positives in week 48 (sentinel)/week 49 (comprehensive) and had a sharp first peak in weeks 51-52/2022, before falling markedly in the first weeks after New Year, then stabilising at about 15% influenza positivity rate among all tested and about 24% among sentinel specimens for the last few weeks (week 4).
- Influenza A viruses have been in great majority until recently, but the proportion and numbers of influenza B detections are rising steadily and was at 15% in week 4. Among the type A viruses, the majority have been subtype H1, but the proportion of H3 are increasing. All circulating influenza B viruses that have been tested for lineage have belonged to the B/Victoria/2/1987 lineage.
- The age group representing school-age children has had the highest proportion of influenza positives throughout the period and has shown rising numbers earlier than the other age groups. The proportion of influenza-like illness (ILI) consultations in primary health care gradually increased from week 48/2022. After a peak in week 52/2022, the proportion decreased to a low level of intensity in week 2/2023, has continued to decline until week 3/2023, and has levelled off since.
- The numbers of hospitalisations and ICU admissions with influenza began to increase around week 46-2022, reaching a peak in week 52-2022. As of week 4-2023, 3653 hospital admissions and 126 ICU admissions have been reported, clearly exceeding numbers reported for the entire preceding season 2021-2022. The weekly number of influenza-associated deaths peaked during weeks 52-2022 – 2-2023, coinciding with the highest rate of all-cause mortality in Norway since 2017.
- Near 16% of all samples received for surveillance have been whole genome sequenced. Both the H1N1 A/Sydney/5/2021 1A.5a.2 lineage and the A/Norway25089/2022 6B.1A.5a.2.1 with the HA P137S substitution have been circulating, but in the last weeks the A/Sydney-lineage are predominating with several separate clusters . The H3N2 viruses are all categorized as 3C.2a.1b.2a.2 belonging to the A/Slovenia/8720/2022 group of viruses with the R299K substitution. All influenza B viruses sequenced were B/Victoria lineage, belonging to the B/Austria/1359417/2021 clade, but several subgroups were detected with several mutation differences.
- Vaccination coverage among risk groups younger than 65 years and health care workers decreased compared to the 2021/2022 season. The coverage rate for individuals above 65 years was 63 %, which is at the same level as last season. The number of distributed doses decreased by 9 % compared to the 2021/22 season. 1.2 million doses intended for use in risk groups and health care workers were distributed.
- Highly pathogenic avian influenza viruses (H5N1, H5N5) belonging to clade 184.108.40.206b continued to be detected in wild birds in Norway. During autumn 2022 there were two outbreaks of H5N1 in commercial poultry flocks. No human cases were detected, and the risk of human infection was assessed as very low.