London — A new meningitis vaccine being rolled out in Chad has resulted in an astonishing 94 percent drop in the incidence of all types of the disease, according to a study being published this week in the Lancet.
The study's author, Brian Greenwood of the London School of Hygiene and Tropical Medicine, described it as one of the most dramatic outcomes he had ever seen in a public health intervention.
Meningitis, particularly the A strain of the of the infection, is a major health problem in the Sahel, the semi-arid zone to the south of the Sahara Desert, which stretches from Senegal in the west to Ethiopia in the east.
Many residents carry the bacteria harmlessly in their throats, but in the early months of each year, the dry, dusty wind known as the Harmattan sweeps down from the desert. Doctors believe that the dryness and dustiness cause irritation and reduced resistance, leading the disease to flare up.
Bad meningitis years bring outbreaks across the Sahel; an outbreak in 1996-1997 is believed to have killed 25,000 people. Existing vaccines were not very effective, especially for young children, and because they did not confer long-lasting protection, they were usually only used during outbreaks, and often arrived too late to be useful.
The new vaccine, known as MenAfriVac, is a conjugate polysaccharide-tetanus toxoid vaccine for type A meningitis, and it was developed specifically for use in the region.
Because its stage one and stage two trials showed it to be effective and safe, and because it was similar to the existing meningitis C vaccine, it was put straight into use without doing wider, stage three trials.
The team from the London School went to the Sahel to study how it worked in practice.
James Stuart, the programme manager for the study, told IRIN that first results, from Burkina Faso, gave some indication that it was working well, but it was not possible to quantify the effect. Meningitis rates in Burkina Faso were already dropping, and, in any case, the disease was not pervasive that year.
But when they turned to Chad, the team had a bit of luck. "What was particular about Chad," he said, "is that one part of the country had been vaccinated and one part hadn't during a meningitis epidemic."
This made for useful experimental and control groups, respectively. But it had not been planned by researchers; the Ministry of Health had simply decided to roll out the vaccine in phases. "We had nothing to do with it, and it was just coincidence that for logistical reasons they decided - unlike Burkina Faso - to do it in phases," Stuart said.
When researchers compared the two areas, the results were striking: In the areas that had received the new vaccine, the incidence of meningitis of all kinds was only 2.5 per 100,000 people, compared with an incidence of 43.6 per 100,000 in the unvaccinated areas.
In fact, no confirmed cases of type A meningitis were found in the vaccinated areas; the only cases confirmed were other strains, against which MenAfriVac offers no protection.
(Meningitis type A is responsible for the vast majority of cases, accounting for the steep decline in overall incidence of the disease.) And this was despite the fact that only people between one and 29 years of age had been vaccinated. Babies and older people still did not seem to be developing the disease.
Stuart says the study offers some explanation: "The other big difference with this new vaccine," he told IRIN, "is that it actually drives the bugs out of the throat. With the old vaccine, people were protected against the disease, but they would still be carriers, and the spread of the bugs would still continue. This one has a double effect, which is why the results have been so dramatic. It looks as if transmission is just stopping."
This creates the alluring possibility that mass vaccination could now make meningitis epidemics in the Sahel a thing of the past.
Still, researchers have a few words of caution. The new vaccine is expected to be longer-lasting than the previous one, but it will take time to know for certain.
Additionally, in a comment published in the Lancet alongside the study, Johannes Elias, of the University of Wurzburg in Germany, warns of the possibility of "serogroup replacement" - the risk that when the meningitis A bacteria are cleared from the throat, other organisms, perhaps other forms of meningitis bacteria, may move in to replace them. He urged better surveillance to warn of this and to monitor continued vaccine effectiveness.
Development of the vaccine was supported by the Meningitis Vaccine Project, a programme of PATH and the World Health Organization.
[This report does not necessarily reflect the views of the United Nations.]