This is an interesting study. It starts with the observation that much flu is transmitted by children. The question then is: if we immunise the children, does that protect everyone else? Of course finding a way to answer this question is not easy, but the researchers went to the semi-closed Hutterite communities. Fair amount of opposition to vaccine in these communities (see the study flow diagram) but enough agreed to participate to enable the trial. I was slightly taken aback that the 'control' group were given Hep A vaccine, but on reflection it didn't seem so bad.
The outcome of interest was laboratory confirmed influenza illness.