Fig. 1

Incidence of ILI per 1000 population-month in each age group. Several surges of ILI symptoms were observed every 2–3 months. Sharp surges of ILI incidence were observed in age group under 2 years (undashed line), which were synchronized with 2–4 years (middle dashed line), 5–14 years (fine dashed line) and > 60 years (rough dashed line). No clear surges were observed in 15–59 years (dotted and dashed lines)