Meningitis can be caused by several species of bacteria, but one of the most important is the meningococcus. In a region of sub-Saharan Africa known as the African meningitis belt, stretching from Senegal in the west to Ethiopia in the east, outbreaks of meningococcal meningitis are regular and deadly. The largest recent outbreak was in 2009 and involved 14 countries, with more than 88 000 suspected cases and more than 5000 deaths.
The study looked at the effectiveness of a vaccine, PsA-TT, which targets the meningococcus sub-type serogroup A, the predominate cause of meningitis in this area. Approximately 1.8 million people aged between one and 29 years received a single dose of the vaccine, also known as MenAfriVac®, in three regions of Chad in December 2011.
Researchers from Africa and Europe measured the incidence of meningitis in Chad during the 2012 meningitis season and the number of people carrying the bacteria that cause the disease in their throat. In the areas where mass vaccination had taken place, no cases of serogroup A meningococcal meningitis were detected during the 2012 season.
The incidence of meningitis of any kind in these areas was less than three cases for every 100 000 people, compared with more than 40 cases per 100 000 in regions where mass vaccination had not been undertaken - a difference of 94 per cent.
Carriage of the disease-causing bacteria was also dramatically reduced: two to four months before vaccination, 32 serogroup A carriers were identified in 4278 people tested through throat swabs. Four to six months after vaccination, only one out of 5001 people tested in the same community was found to be carrying serogroup A.
Study author, Professor Sir Brian Greenwood from the London School of Hygiene and Tropical Medicine (who has studied meningococcal meningitis in Africa since the early 1970s), said: "This is one of the most dramatic outcomes from a public health intervention that I have seen during a long career of research in Africa. There are now real prospects that the devastating effects of this infection in Africa can be prevented.''
Infants, children and young adults are most at risk of meningitis, an inflammation of the membrane around the brain and spinal cord that can cause death or disability, including deafness, paralysis and limb infection leading to amputation. The bacteria are passed from person to person through droplets from the throat via coughing, sneezing or other close contact.
The affordable PsA-TT vaccine was developed by the Meningitis Vaccine Project - a partnership between WHO and PATH - with funding from the Bill & Melinda Gates Foundation. The vaccine was first rolled out in Burkina Faso in 2010, but evaluating its impact there was difficult as there were few serogroup A infections occurring at the time the vaccine was introduced.
The evaluation study, funded by the Bill & Melinda Gates Foundation, the Wellcome Trust and Médecins sans Frontières, is the first to demonstrate unequivocally the high level of effectiveness of PsA-TT in preventing serogroup A epidemic meningococcal meningitis disease and carriage of the bacteria.
Dr Jean-Marie Okwo-Bele, Director of the WHO Department of Immunization, Vaccines and Biologicals, said: "This is an extremely encouraging signal for those countries that have yet to introduce the vaccine. We are not even halfway done with introducing this revolutionary new vaccine across the meningitis belt of Africa, yet we already have extraordinary results."
The authors say several more years of surveillance are needed to establish how long the vaccine remains effective in preventing epidemics and whether other groups of meningococci replace the ousted serogroup A meningococcus.