Here's an interesting paper examining the effect of flu control measures during the 1918/19 pandemic. You can argue about whether flu vaccination works or not, and whether Tamiflu is effective, but neither were available in 1918 so they had to rely on good old fashioned methods - basically stopping people congregating in large numbers, at school or sports matches. And as usual in communicable disease control the prompter and more thorough you are, the more effective the control. Late and half hearted is useless!
But note also a couple of outliers in the data. I love outliers - they can tell you much if pondered carefully.