The most common N. meningitis strain in the United States is serotype B, followed by C. Strains A, B, C, W135, X and Y have all been associated with epidemics with varying geographical distribution. From 1998 to 2007, serotype B was the most common strain in the U.S. with a median infection age of 12. This serotype was also the most common for much of the time between 2008 and 2013 and was the most common serotype isolated from infections involving the college-age population. While infection rates have declined over the past 15 years, the mortality rate has remained around 11 percent for all cases. Fortunately, a few approved vaccines have been around for a while that provide immunity for most serotypes of N. meningitidis, thus keeping the mortality rate lower than it probably would have been.