Vaccinations work by exposing patients to antigens from pathogens which induces protective immunity. There are two main types of meningococcal vaccines - the meningococcal polysaccharide vaccine (MPSV) approved for those over 2 years old, and the meningococcal conjugate vaccine (MCV) approved for those 2 to 55 years old. The MCV is routinely recommended for adolescents, college freshmen living in dorms, and adults at increased risk, while the MPSV is only recommended for those over 56 at increased risk. Contraindications include severe allergic reactions or moderate to severe acute illness.