a, Number of ILI cases and percentage of outpatients with ILI between 6 October 2019 and 21 January 2020 (2019W40 to 2020W03). The vertical columns scaled on the left y axis report the weekly ILI case numbers and the colour blocks in the columns represent the different age groups of the patients with ILI. The dashed line shows the percentage of outpatients with ILI and is scaled on the right y axis. b, Number of ILI cases that occurred during the winter influenza season of the past three years. c, Percentage of ILI during the winter influenza season of the past three years. b,c, Data from week 40 to week 12 of the following year are presented, except for 2019–2020 as the ILI surveillance was suspended to handle the COVID-19 epidemic.