Figure 4 is a line graph showing the national percentage of participants with ILI (defined as fever and cough) each week (ending Sunday) from 2009 to 2015, stratified by vaccination status (vaccinated versus unvaccinated with the seasonal influenza vaccine). For the majority of survey weeks there was a higher percentage of unvaccinated participants with ILI, as compared to vaccinated participants. Peak ILI activity for 2015 was during the week ending 23 August (5.0% in the unvaccinated group and 3.8% in the vaccinated group). However, divergence between the vaccinated and unvaccinated participants’ ILI percentages was highest during the week ending 30 August.