Meningococcal disease is caused by the Gram-negative bacterium Neisseria meningitidis (the meningococcus). It remains a significant public health issue globally causing both endemic and epidemic disease in developed and developing countries.
Approximately 10% of humans harmlessly carry N. meningitidis in the oronasopharynx. On very rare occasions the bacteria may cross the epithelium and enter the blood stream causing sudden onset of sepsis and or meningitis with high complication and case fatality rates, even with appropriate antibiotic treatment.
A limited number of strains cause the majority of invasive disease and, in normally healthy individuals, these practically always express a protective polysaccharide capsule on their cell surface.
There are 12 capsular serogroups, of which A, B, C, W, X and Y cause the vast majority of invasive meningococcal disease worldwide.
Polysaccharide-based vaccines target the capsule and so are serogroup-specific.
Plain (unconjugated) polysaccharide vaccines were developed first and have been used in control of serogroup A epidemics in sub-Saharan Africa and for controlling serogroup C disease in the military and college students. Associated limitations include poor immunogenicity in young children, hyporesponsiveness with repeat doses, inability to induce immune memory and lack of an effect on carriage.
Conjugated polysaccharide vaccines have none of these limitations and, most importantly, significantly reduce carriage. Therefore, large scale vaccination of cohorts with high carriage (catch-up campaigns) are highly effective in inducing herd protection.
Serogroup C conjugate vaccines have been hugely successful in dramatically reducing disease in the countries that have instigated immunisation programs together with appropriate catch-up campaigns.
Meningococcal quadrivalent conjugate vaccines are now being implemented into schedules.
With the development and introduction of a meningococcal serogroup A conjugate vaccine, serogroup A disease has disappeared from those sub-Saharan countries who have implemented campaigns.
The serogroup B polysaccharide is poorly immunogenic and so broad coverage protein-based serogroup B vaccines have been developed.