Meningitis epidemics caused by the pathogen Nesseiria meningitis (or meningococcus) provoke high mortality in children and young people under 20 years of age in sub-Saharan Africa. They rage during periods of drought from January to April, in the area known as the "Meningitis Belt" (see Map)
Two types of vaccine are currently on the market: the polysaccharide vaccine and the conjugated vaccine. The polysaccharide form, elaborated from a sugar present at the surface of the meningococcus provides only partial, temporary immunity in very young children (2).
The conjugated vaccine, however, is based on a combination between this same sugar and an antigenic protein and renders an effective durable immunity. It could therefore be used in a routine preventive vaccination campaign. But it is costly. Its high price, at 11 to 22 euros per dose, means it remains inaccessible for African countries. These have available only the polysaccharide vaccine, less expensive at 0.3 to 0.5 euro, but which, because of its low immunity capacity, is generally kept only for emergency vaccinations, when epidemics occur.
IRD researchers have been running a follow-up study of the people of the Niakhar region (located 150 km from Dakar, in Senegal) for more than 40 years. They looked into the question of the ability of the polysaccharide vaccine to prevent the occurrence of meningitis cases during epidemics in the course of subsequent years. To do that, they made use of results of two vaccination campaigns conducted with this vaccine by the Senegalese health services. One of these was run in 1996 in 8 of the 30 villages that make up the Niakhar region, the other in 1999, in the whole of the study area, hit be a series of epidemics between 1998 and 2000.
Using quarterly censuses carried out since 1983, the researchers collected then analysed data on children's state of health and vaccine status. They could therefore make comparisons between the different villages regarding the number of cases of meningitis that broke out, in particular in the years that followed the 1996 vaccination campaign (3).
Their investigation revealed that in the villages vaccinated at that date, two to three times fewer subjects were recorded as suffering from the disease. The polysaccharide vaccine could help avoid up to 72% of cases in subsequent epidemics.
These results speak in favour of adopting a strategy of routine vaccination using this vaccine -- whenever no other vaccine is available -- in order to prevent meningococcus meningitis epidemics in this region of Africa. However, a conjugated vaccine which should be sold at less than 1$ (0.80 euro) per dose, is under development. Thanks to the full, durable immunity it confers right from early childhood, it is an ideal tool for a preventive vaccination strategy. Clinical trials initiated in 2006, notably in Niakhar, are continuing for a further 2 years in order to make sure that no undesirable side-effects occur after administration.
(1) This research work was conducted by IRD service unit US 009 (Suivi démographique, épidémiologique et environnemental), and research units UR 010 (Santé de la mère et de l'enfant en milieu tropical: épidémiologie et génétique périnatale) and UR 145 (VIH/SIDA et maladies associées).
(2) The polysaccharide vaccine does not prevent a subject from being a carrier of meningococcus or from transmitting it, but it stimulates individual immunity with a benefit for the whole of the community (collective immunity) according to a yet unknown mechanism. In this study, the reasoning is therefore for the whole of the group and not for the individuals taken separately.
(3) A case of meningitis was defined by the sudden appearance, between January and May for each of three years (1998, 1999, 2000), of a severe fever associated with intense migraines, vomiting and stiffness at the nape of the neck. The epidemic threshold was determined according to WHO (World Health Organization) criteria. It is reached when 2 new cases are detected in the course of the same week or when the number of cases doubles in 3 weeks.
Reference: Jean-Philippe Chippaux, Aldiouma Diallo, Adama Marra, Jean-François Étard -- Impact of previous immunisation on the incidence of meningococcal disease during an outbreak in a Sahelian area of Senegal, Vaccine, vol. 25 : 1712-1718 (2007)
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