Meningococcal meningitis is the only form of bacterial meningitis that causes epidemics.
Serogroup A meningococci have almost exclusively been associated with the most explosive epidemics in Africa and Asia, while B and C can be found in Western Europe. Certain strains of meningococci are more virulent than others and more likely to cause outbreaks. Travel, migration, poor living conditions and large population displacements (refugees) facilitate the circulation of virulent strains.
According to the WHO, major epidemics occur most frequently but are not restricted to an area in the African meningitis belt that extends across Africa from Ethiopia in the east to Senegal in the west and includes all or part of 15 countries. Epidemics usually start there in the dry season (December to June) and end promptly when the rainy season begins. The incidence rate in these countries varies from 10-20 per 100,000 inhabitants.
The spread of a single clone of serogroup A N. meningitidis may be linked to the most recent pandemic originating from China and, passing through Nepal and Northern India, entering the Arabian peninsula, where it caused a large outbreak in Mecca in 1987 at the end of the pilgrimage period. From there the disease spread to many other African countries. Infections due to this clone frequently present as sepsis with a high risk of fatal outcome. In 1998, the outbreaks occurring in Chad and Cameroon accounted for about 30% of the total cases reported that year.
Epidemics caused by serogroup B meningococci are less intense than those caused by serogroup A, but may extend over a more prolonged period, often for many years. In most European countries group B meningococcal disease, against which no vaccine is available to date, is common.
1996 saw an outbreak in northwest Spain, in a province called Galicia. A mass vaccination campaign, using polysaccharide vaccine, was carried out for the total population aged 18 months to 19 years in order to bring the epidemic under control.
In temperate climates (northern hemisphere), the disease is endemic causing a steady number of sporadic cases or small clusters with a seasonal increase in winter and spring, beginning in December-January and culminating in March-April.