Fig. 5.

Indirect neutralization by HA stalk specific antibodies though ADCC. Serum antibodies ability to induce HA stalk specific ADCC, measured as frequencies of NK cells expressing (B) CD107a or (C) IFNγ with NK cell activation assay using cH9/1 protein, and (D) by measuring luciferase activity with reporter assay (promega kit) using cH9/1 N3 virus. Four groups of healthcare workers (Group 1; G1, Group 2; G2, Group 3; G3 and Group 4; G4) were retrospectively selected according to their year of birth (G1; 1947–1952, G2; 1958–1960, G3; 1969–1972 and G4; 1978–1980). Serum from samples were collected from the four groups of healthcare workers pre- and 21 days post vaccination. Each data point represents a single individual. The geometric mean titre (GMT) ± 95% confidence intervals are shown. # Indicates statistical significance (p < 0.05) from pre to post vaccination.