Influenza Virological and Epidemiological season report 2022-2023
Report
|Published
The 2022-23 season started early, crossing the outbreak threshold of 10 % positives in week 48 (sentinel)/week 49 (comprehensive) and had a sharp first peak in weeks 51-52/2022 with 46 % positives in the sentinel and 25 % positives in the comprehensive surveillance.
Key message
- 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 in the 3C.2a1b.2a.2 group predominated.
- Protective antibody levels against A/Victoria/2570/2019(H1N1) were at a moderate level in August 2022. However, antibody levels in the youngest age group were very low, suggesting a high degree of susceptibility. There was significantly less antibodies against the A/Norway/25089/2022 strain, which became a prominent subvariant during the H1 outbreak seen in early winter. Protective antibody levels against A/Darwin/9/2021 (H3N2) were low in the general population. Antibody levels against recent B/Victorialineage virus was at a low level, especially in the younger age groups.
- The vaccine coverage in the national immunisation programme decreased from the previous season. Approximately 1.2 million doses were distributed to risk groups and health care personnel, and the number of discarded doses was estimated to 109.000. According to the national immunisation registry SYSVAK, the vaccine coverage among persons 65 years or older was at least 62 percent. The estimated number of doses used (private market included) decreased by 8 % compared to the preceding 2021/22 season.
Both registry-based coverage rates and self-reported vaccination has decreased in several groups compared to the 2021/22 season. This is most notable among the risk groups 18-64 years and health care personnel. - The 2022-23 season started early, crossing the outbreak threshold of 10 % positives in week 48 (sentinel)/week 49 (comprehensive) and had a sharp first peak in weeks 51-52/2022 with 46 % positives in the sentinel and 25 % positives in the comprehensive surveillance. The positivity rate fell markedly in the following few weeks before it recovered and went through two smaller peaks in weeks 6 and 12, respectively. After this, the numbers declined gradually, falling below 10 % positives in overall testing in week 15 and in sentinel testing in week 18. The positivity rate has been very low (below 1%) since midsummer, but with sporadic detections in every week.
- Influenza A(H1N1) viruses predominated in the first and largest peak around New Year. With subsequently declining numbers, the frequencies of H1N1 and H3N2 also became more even. Influenza B/Victoria lineage viruses started to rise after New Year, passed influenza A in week 8, and were predominant in the last wave that peaked in week 12. Since midsummer, influenza A viruses have again been in majority among the few detections with a majority of them being H1N1 during summer and H3N2 during early autumn. 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 40/2022, with a rapid increase from week 48/2022, crossing the epidemic threshold the week after. It peaked in week 52/2022, several weeks earlier than normal. After a steep decrease until week 3/2023, the decrease was more gradual, including two minor peaks corresponding to the pattern in the virological
surveillance. It crossed below the epidemic threshold in week 14/2023, resulting in a 14-week-long influenza outbreak, two weeks longer than average, before decreasing further down toward and through the summer. - 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 39-2023, 5509 hospital admissions have been reported. Between week 40-2022 and 20-2023, 191 ICU admissions were reported. Both of the number of hospitalisations and ICU admissions clearly exceed numbers reported for the preceding season 2021-2022. Hospitalisation rates among the 0-4- and 5-14-year-olds were at a higher level compared to all previous seasons since 2017-18, since when surveillance data is available.
- Nine influenza outbreaks in long-term care facilities were notified throughout the season.
- 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. A total of 25,704 cases of influenza have been laboratory confirmed, out of 284,663 patients tested during this season. The number of tests is considerably lower than the more than 600,000 performed during the preceding season, when large-scale testing for COVID-19 screening with less clinical indication was driving numbers upward. However, more people were tested than in earlier seasons when the number typically was around 200,000 patients.
- 30% (1378/4546) of all influenza positive samples received for surveillance have been whole genome sequenced. Both the H1N1 A/Sydney/5/2021 6B.1A.5a.2 lineage and its A/Norway/25089/2022 6B.1A.5a.2.1 sublineage with the HA P137S substitution have been circulating, but by mid-season the A/Sydney-lineage viruses predominated 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 some mutation differences and dominated the late season.
- A total of 496 influenza samples have been examined for resistance to antivirals. Only a single H1N1 virus had resistance mutations to the neuraminidase inhibitor Oseltamivir, resistance was developed through antiviral treatment. All viruses are still resistant to adamantanes, but not to neuraminidase or polymerase inhibitors and are thus sensitive to treatment with Tamiflu® and XOFLUZA®
- Highly pathogenic avian influenza viruses (H5N1, H5N5) belonging to clade 2.3.4.4b continued to be detected in wild birds in Norway. During autumn 2022 there were two outbreaks of H5N1 in commercial poultry flocks. In the summer of 2023, there was a mass mortality event among seagulls (particularly black-legged kittiwakes) along the northern coastline of Norway, caused by HPAI H5N1. This virus was also detected in a young red fox found dead in the same area. No human cases have been detected, and the risk of human infection has been assessed as very low.
