Meningococcal carriage in the African meningitis belt

Meningococcal vaccines and herd immunity: lessons learned from serogroup C conjugate vaccination programs

Effective vaccines provide direct protection to immunized individuals, but may also provide benefits to unvaccinated individuals by reducing transmission and thereby lowering the risk of infection. Such herd immunity effects have been demonstrated following the introduction of meningococcal serogroup C conjugate (MCC) vaccines, with reductions in disease attack rates in unimmunized individuals and significantly lower serogroup C carriage attributable to the vaccine introduction. In the UK, targeting teenagers for immunization was crucial in maximizing indirect effects, as most meningococcal transmission occurs in this age group. Questions remain regarding the duration of herd protection and the most appropriate long-term immunization strategies. The magnitude of the herd effects following MCC vaccination was largely unanticipated, and has important consequences for the design and evaluation of new meningococcal vaccines.

Global epidemiology of meningococcal disease

As reviewed in this paper, meningococcal disease epidemiology varies substantially by geographic area and time. The disease can occur as sporadic cases, outbreaks, and large epidemics. Surveillance is crucial for understanding meningococcal disease epidemiology, as well as the need for and impact of vaccination. Despite limited data from some regions of the world and constant change, current meningococcal disease epidemiology can be summarized by region. By far the highest incidence of meningococcal disease occurs in the meningitis belt of sub-Saharan Africa. During epidemics, the incidence can approach 1000 per 100,000, or 1% of the population. Serogroup A has been the most important serogroup in this region. However, serogroup C disease has also occurred, as has serogroup X disease and, most recently, serogroup W-135 disease. In the Americas, the reported incidence of disease, in the range of 0.3-4 cases per 100,000 population, is much lower than in the meningitis belt. In addition, in some countries such as the United States, the incidence is at an historical low. The bulk of the disease in the Americas is caused by serogroups C and B, although serogroup Y causes a substantial proportion of infections in some countries and W-135 is becoming increasingly problematic as well. The majority of meningococcal disease in European countries, which ranges in incidence from 0.2 to 14 cases per 100,000, is caused by serogroup B strains, particularly in countries that have introduced serogroup C meningococcal conjugate vaccines. Serogroup B also predominates in Australia and New Zealand, in Australia because of the control of serogroup C disease through vaccination and in New Zealand because of a serogroup B epidemic. Based on limited data, most disease in Asia is caused by serogroup A and C strains. Although this review summarizes the current status of meningococcal disease epidemiology, the dynamic nature of this disease requires ongoing surveillance both to provide data for vaccine formulation and vaccine policy and to monitor the impact of vaccines following introduction.

Epidemic meningitis due to Group A Neisseria meningitidis in the African meningitis belt: a persistent problem with an imminent solution

Epidemic meningitis in Africa remains an important and unresolved public health problem. Bacteriologic and epidemiologic data collected over the past 30 years have consistently established the importance of Group A Neisseria meningitidis as the dominant etiologic agent. The meningococcal Group A capsule is the major virulence factor; it is a polysaccharide comprised of a repeating unit of partly O-acetylated alpha-1,6-linked N-acetylmannosamine phosphate. Meningitis epidemics occur annually during the dry season (January to May) and stop with the first rains. Until now, control of these meningitis epidemics has relied on a reactive vaccination strategy with polysaccharide vaccines that is logistically complicated and has not put an end to recurrent epidemics. A meningococcal A conjugate vaccine (MenAfriVac) has been developed and tested in Phase II clinical trials in Africa. The vaccine has been shown to be safe and to generate a sustained immunologic response with functional antibody 20 times higher than that seen with polysaccharide vaccine. Widespread use of such a vaccine is likely to generate herd immunity and to put an end to Group A meningococcal epidemics.

Quadrivalent meningococcal ACYW-135 glycoconjugate vaccine for broader protection from infancy

Invasive meningococcal disease is a global public-health concern, with infants and adolescents bearing the majority of the disease burden. Vaccination is the most rational strategy to prevent meningococcal disease. Control of serogroup C disease has largely been achieved by the introduction of glycoconjugate meningococcal C vaccines, initially in the UK in 1999, and subsequently in several other countries. The recent licensure of a quadrivalent glycoconjugate vaccine against serogroups A, C, Y and W-135 in the USA and Canada has broadened protection against Neisseria meningitidis in 2-55 year olds. The investigational quadrivalent meningococcal serogroup A, C, Y and W-135 glycoconjugate vaccine (MenACYW-CRM197), which is immunogenic from infancy, has the potential to extend protection to the most vulnerable age group. This article discusses this novel quadrivalent vaccine formulation and its potential to control invasive disease caused by N. meningitidis serogroups A, C, Y and W-135.

Meningococcal carriage and disease--population biology and evolution

Meningococcal disease occurs worldwide with incidence rates varying from 1 to 1000 cases per 100,000. The causative organism, Neisseria meningitidis, is an obligate commensal of humans, which normally colonizes the mucosa of the upper respiratory tract without causing invasive disease, a phenomenon known as carriage. Studies using molecular methods have demonstrated the extensive genetic diversity of meningocococci isolated from carriers, in contrast to a limited number of genetic types, known as the hyperinvasive lineages, associated with invasive disease. Population and evolutionary models that invoke positive selection can be used to resolve the apparent paradox of virulent lineages persisting during the global spread of a non-clonal and normally commensal bacterium. The application of insights gained from studies of meningococcal population biology and evolution is important in understanding the spread of disease, as well as in vaccine development and implementation, especially with regard to the challenge of producing comprehensive vaccines based on sub-capsular antigens and measuring their effectiveness.

Relationships between climate and year-to-year variability in meningitis outbreaks: a case study in Burkina Faso and Niger

Background

Every year, West Africa is afflicted with Meningococcal Meningitis (MCM) disease outbreaks. Although the seasonal and spatial patterns of disease cases have been shown to be linked to climate, the mechanisms responsible for these patterns are still not well identified.

Results

A statistical analysis of annual incidence of MCM and climatic variables has been performed to highlight the relationships between climate and MCM for two highly afflicted countries: Niger and Burkina Faso. We found that disease resurgence in Niger and in Burkina Faso is likely to be partly controlled by the winter climate through enhanced Harmattan winds. Statistical models based only on climate indexes work well in Niger showing that 25% of the disease variance from year-to-year in this country can be explained by the winter climate but fail to represent accurately the disease dynamics in Burkina Faso.

Conclusion

This study is an exploratory attempt to predict meningitis incidence by using only climate information. Although it points out significant statistical results it also stresses the difficulty of relating climate to interannual variability in meningitis outbreaks.