True, many will become infected naturally. The problem is that the antigenic shift of influenza is affected by transmission rate. That is, the more times a viru

True, many will become infected naturally. The problem is that the antigenic shift of influenza is affected by transmission rate. That is, the more times a virus gets passed from person to person, the more it can mutate. The vaccines that are developed are based on a "normal" rate of antigenic shift. If large groups of people try to get infected, there is a risk that they will throw off this rate.