Figure 4 is a line graph showing the national proportion of participants with ILI (defined as fever and cough) each week (ending Sunday) from 2010 to 2012, stratified by vaccination status (vaccinated versus unvaccinated with the seasonal influenza vaccine). For the majority of survey weeks there is a higher proportion of unvaccinated participants with ILI, as compared to vaccinated participants. Peak ILI activity for 2012 was in mid-July (4.9% in the unvaccinated group). However, the divergence between ILI proportion for the vaccinated and unvaccinated participants’ was highest in early June and mid-August. Peak ILI activity for 2011 was in mid-August (4.1% in the unvaccinated group). However, the divergence between ILI prevalence in vaccinated and unvaccinated participants was greatest between late May and early June. Peak weekly ILI prevalence in 2011 was similar to peak weekly ILI prevalence in 2010 (4.2% among unvaccinated participants).