Final answer:
The Haemophilus influenzae type b (Hib) vaccine has significantly reduced cases of meningitis in countries where it is used, particularly among children. Neisseria meningitidis also has a vaccine which has lowered the incidence of epidemic meningitis, notably on college campuses and in Africa's meningitis belt.
Step-by-step explanation:
The causative organism of meningitis that has been significantly reduced with a vaccine is Haemophilus influenzae type b (Hib). Cases of meningitis due to H. influenzae serotype b, once a primary cause of meningitis in children between 2 months and 5 years of age, have decreased in countries where the effective Hib vaccine is used. Immunization against H. influenzae causes a significant drop in the incidence of meningitis by establishing herd immunity and protecting against invasive disease.
Neisseria meningitidis, which causes epidemic meningitis cases, particularly in college campuses, also has a vaccine available. Vaccination programs, like the one in Africa's meningitis belt that uses a new serogroup A meningococcal conjugate vaccine, have dramatically reduced cases of Neisseria meningitidis meningitis as well.