Fig. 4

HAI titers against A/Darwin/9/2021(H3N2) in sera from 2021 and 2022 in different age groups. A Reverse cumulative plots were generated from the HAI titers against A/Darwin/9/2021 from 2021 and 2022 for the age groups 0–4 years, 5–14 years, 15–24 years, 25–59 years and 60 + years. The dotted line indicates 50% protective HAI titer of 40. B Number of detected influenza A infections per 100.000 individuals in the different age groups were extracted from the Norwegian Laboratory Database for the period week 1 2022 to week 26 2022. C Reverse cumulative plots of HAI titers in 119 sera collected in 2021 from one reference lab tested against A/Darwin/9/2021 in both 2021 and 2022 to verify reproducibility. Significant differences were calculated by a two-tailed Mann–Whitney test. ***p < 0.001 and ns = no significant difference