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The 2018-2019 influenza season, Norway
- Seroepidemiology data from August 2018 indicate that immunity in Norway against circulating influenza A(H1N1) and A(H3N2) viruses was quite strong. Also for the B/Yamagata-lineage virus that caused the preceding winter’s influenza outbreak, there was a marked increase in people with antibody at protective levels.
- Added to this came the immunity due to the subsequent influenza vaccination campaign in the autumn. Rates of vaccination were raised considerably that year.
- The seasonal influenza outbreak began in week 52. It reached medium intensity, a level it remained at for three weeks. The flu peak was reached in week 7. The outbreak was over in week 13 and had a duration of 13 weeks.
- Influenza A(H1N1) virus predominated, constituting approximately 60% of detections. The remainder was mainly A(H3N2) virus, with unusually few (1%) influenza B viruses. Nevertheless, among the elderly, A(H3N2) infection was more likely than A(H1N1).
- As in earlier A(H1N1)pdm09 dominated seasons, young children were more likely than others to be diagnosed with influenza infection.
- Among the few influenza B viruses, the B/Victoria-lineage was slightly more frequent than the B/Yamagata-lineage. Whereas B/Yamagata-lineage viruses were more common in the beginning, only B/Victoria-lineage viruses have been observed since week 23.
- The majority of the H1N1 viruses were characterised as 6B.1A5 A/Switzerland/3330/2017 viruses, but during the summer months a new subgroup under 6B.1A5 emerged possessing a number of substitutions in HA (K130N;K160M;T216K;E235D;H296N and V321I).
- There has been a number of different subgroups of H3N2 viruses circulating also this season, but the main group has been the 3C.2a1b viruses. During the summer, viruses in this group carrying the Q197R together with K207R have become more prominent.
- All influenza B/Yamagata-lineage viruses were HA clade 3 viruses. Two influenza B-Yamagata viruses have been detected that possess a large number of substitutions in both HA and NA.
- The African triple deletion variant has been most prevalent among the B/Victoria-lineage viruses this season.
- About 4 400 patients with laboratory-confirmed influenza were hospitalised. Compared with the previous season, there were far fewer hospitalisations, less influenza patients requiring intensive care unit (ICU) admission, and fewer weeks with excess all-cause mortality in the population.
- The highest influenza-associated hospitalisation rates were found in the elderly (60 years or older) and in young children (0-4 years). There were fewer hospitalised elderly this season. However, in young children, the hospitalisation rate was relatively high.