Although several bacteria can cause meningitis, Neisseria meningitidis is the one with the potential to cause large epidemics. This gram-negative bacterium is carried in the human nasopharyngeal mucosa, transmitted from person to person by upper respiratory secretions, and occasionally crosses the mucosal membrane and enters the bloodstream causing various clinical syndromes. Acute meningitis is the most frequent.
Twelve serogroups of Neisseria meningitidis have been identified, six of which (A, B, C, W135, X and Y) can cause epidemics. The highest disease burden is reported from sub-Saharan Africa where for over a century, major meningococcal meningitis epidemics have occurred every 10–12 years in the African meningitis belt, a savannah region extending from Senegal to Ethiopia. The majority of these epidemics are caused by group A and incidence rates can be as high as 500 cases per 100 000 population, affecting primarily the under 30 age group. While group A predominates through Africa and Asia, groups B and C are responsible for the majority of cases in Europe, the Americas, Australia and New Zealand.
Meningococcal polysaccharide vaccines have been used to control the disease for over 30 years. They are available in either bivalent (groups A and C), trivalent (groups A, C and W), or tetravalent (groups A, C, Y and W135) forms. For group B, however, polysaccharide vaccines cannot be developed due to antigenic mimicry with the meningococcal B polysaccharide in human neurologic tissues. Consequently, vaccines against B used in particular in Cuba, New Zealand and Norway have been outer membrane proteins and strain-specific, tailored to control epidemics caused by a particular strain. Promising meningococcal group B protein vaccines are in late stages of development.
Meningococcal conjugate vaccines against group C have been widely used to control the disease since 1999. Tetravalent A, C, Y and W135 conjugate vaccines have been licensed since 2005 for use in children and adults in Canada, the United States of America, and Europe.