Why are there two different meningococcal vaccines (Meningococcal ACWY and Meningococcal B)?

The Meningococcal ACWY immunization protects against four groups of bacteria (A, C, W, and Y) that commonly cause disease. The Meningococcal B vaccine protects against the B group of bacteria that commonly causes disease. It is completely safe to receive both immunizations during the same visit.