The "50% efficacy" doesn't indicate that the vaccine provides 50% effectiveness in blocking a particular influenza; rather it stems from the fact that the parti

The "50% efficacy" doesn't indicate that the vaccine provides 50% effectiveness in blocking a particular influenza; rather it stems from the fact that the particular influenza(s) being targeted might not be the ones passing through an area. "The flu" is an umbrella term for a number of different infectious strains (that mutate), and the object of the game is to detect early on which virulent, dangerous and uncommon strains (already active elsewhere) are most likely to spread widely and vaccinate against them. Frequently, less-virulent strains to which a large percentage of the population has already been exposed will be ignored (hitting somebody with hundreds of strains to develop immunity to simultaneously reduces the effectiveness of the vaccine for all of them). And there's always the chance that a particularly nasty strain may become epidemic after this year's vaccine has been formulated and distributed (in which case, there may or may not be an effective vaccine available in time for people to develop immunity to that strain). So even though you have been effectively vaccinated against the targeted strain(s), you can still contract other strains. And you may have a miserable time dealing with something you did catch, which can keep you from thoproughly enjoying the fact that you didn't catch something that may have been many, many times worse.