London School of Hygiene & Tropical Medicine

The proportion of people who carry the bacteria Neisseria meningitidis, the meningococcus which can cause meningitis, varies widely between countries and over time according to the findings of a large-scale study published in The Journal of Infectious Diseases.

The African Meningococcal Carriage Consortium (MenAfriCar), funded by the Wellcome Trust and Bill & Melinda Gates Foundation, has investigated patterns of meningococcal meningitis and carriage in seven countries in the sub-Saharan 'meningitis belt', extending from Senegal in the west through Mali, Ghana, Niger, Nigeria and Chad, to Ethiopia in the east. The consortium is co-ordinated by Brian Greenwood, Professor of Clinical Tropical Medicine at the London School of Hygiene & Tropical Medicine.

Outbreaks of meningococcal meningitis are frequent in this region, with infants, children and young adults most at risk of death or disabilities including deafness and paralysis. The most recent major outbreak in 2009 involved 88,000 suspected cases and more than 5,000 deaths. However, most people who are infected with meningococcus show no symptoms although the bacteria are present, being carried in the throat and so study of carriage as well as meningitis is necessary to fully understand the epidemiology of infection.

During a period of three years, the researchers collected nearly 50,000 throat swabs from volunteers in seven countries, including three  (Mali, Niger and Chad) where throat swabs were obtained before and after vaccination of the population with a new meningococcal conjugate vaccine called MenAfriVac®.

The results also showed that the overall prevalence of meningococcal carriage in the African meningitis belt is lower than in industrialised countries, perhaps explaining in part why epidemics occur frequently in this region as population susceptibility may be high. The frequency of carriage was highest in older children and young adults. The investigators found marked differences in the frequency of different strains across the meningitis belt and showed that these could change rapidly over time, even in the absence of vaccination.

In Mali and Niger it was not possible to test the impact of MenAfriVac® because the prevalence of the epidemic strain was low before the vaccine was introduced. However, following the vaccination of almost two million people in Chad in 2012, there was a dramatic reduction in the incidence of all cases of meningitis by 94% and in carriage prevalence of the epidemic strain, serogroup A meningitis, by 98% while an epidemic persisted in unvaccinated parts of the country. These results were published in The Lancet in September 2013. The following year, the rest of the country was vaccinated and the epidemic was completed halted; these findings were published earlier this year.

Lead author Professor Brian Greenwood said: "The findings from this large and complicated study have shown that the epidemiology of meningococcal infection is very different in the African meningitis belt than in industrialised countries, and this may help us understand why epidemics are so frequent in this region. Although MenAfriVac® has been very effective in preventing outbreaks of serogroup A meningococcal meningitis other serogroups can cause epidemics, so surveillance of both cases and carriage needs to continue"

The diversity of meningococcal carriage across the African meningitis belt and the impact of vaccination with a group A meningococcal conjugate vaccine

Publication

• The MenAfriCar consortium, The Diversity of Meningococcal Carriage Across the African Meningitis Belt and the Impact of Vaccination With a Group A Meningococcal Conjugate Vaccine. Journal Infectious Diseases.(2015) doi: 10.1093/infdis/jiv211