By nearly everyone’s admission, this year’s influenza vaccine has been a colossal flop. In any given year, flu shot effectiveness in the United States varies widely anyway, but this year’s estimates point to rock-bottom effectiveness of 10%. The previous low over the past five years was an estimated 19% for the 2014-2015 influenza season, when public health researchers concluded that the shot “offered little protection” against the predominant influenza strain.