Epidemics frequently occur in highly mobile populations in which new “recruits” enter the epidemic focus as older residents leave. In such settings, vaccination programs must occur frequently or be continuous. The model for this setting is the military recruit camp, but similar situations have existed during outbreaks of group A disease in the U.S. Pacific Northwest and in Africa.
Endemic disease is concentrated in infants and very young children who do not respond to antigens in the existing vaccine(s) or in whom the duration of immunity is short.
In general, public health policy has resulted in the utilization of a staggered, case-triggered, pulse-vaccination model that has been shown to be only partially effective. When vaccination has been maintained for long periods of time, as in Finland and in the U.S. military, meningococcal disease has been reliably controlled. Such an approach is quite expensive and involves a level of medical infrastructure that is not present in most countries, even the most highly developed. Thus, even though safe and effective for certain age groups, the currently available vaccines have been used infrequently in the developed world. Their potential benefits in developing countries are difficult to predict from studies conducted in developed countries or from their even more limited use in developing countries. Certain improvements (described below) would undoubtedly render existing meningococcal vaccines more valuable in combating meningococcal diseases in developing countries.
The meningococcus is a common commensal of human nasopharyngeal mucous membranes. It spreads from person to person by aerosol droplets. Its colonization of the nasopharynx is often unnoticed in terms of causing disease, and it may persist there for up to 18 months. Factors that influence acquisition are poorly understood.
Only encapsulated N. meningitidis strains can cause disseminated disease. The capsule is composed of linear polymers of various sugars that have been chemically characterized. Of the 12 defined capsular groups, only groups A, B, C, Y, and W135 are clinically important. Because the capsular polysaccharides are both chemically and immunologically distinct, a polyvalent approach to vaccine development using these polysaccharides has been necessary. Only the group B polysaccharide is not effectively immunogenic in humans.
The outer membrane of the meningococcus also contains protein and lipooligosaccharide (LOS) antigens that, collectively, comprise the serotypes. The same serotype antigens may be found in different serogroups, although those of group A strains are largely distinct from those of the other serogroups. To date, at least five protein serotypes and five LOS serotypes have been associated with epidemic disease. Endemic disease, in contrast, is caused by strains of much greater serotype diversity. However, vaccines containing noncapsular surface antigens are the only practical option for a group B vaccine.