More than 100 people have died from measles in the Chadian capital N'djamena and the surrounding southern provinces and up to 24,000 people could be infected with the virus, Medecin Sans Frontieres (MSF) said on Friday.
Since the beginning of the year, there have been 6,000 reported cases nationwide, but MSF said the real figure could be two to three times higher.
Officially 115 have died from the disease and MSF said that in the paediatric wards of some hospitals, 50 percent of mortalities in April were as a result of measles.
The international aid agency said it was trying to establish whether the worst was over.
"We have to collect epidemiological data to define if the epidemic has already reached its peak or not," Kate Alberty, an MSF epidemiologist said in a statement issued on Friday.
MSF staff are treating patients in five health centres in N'djamena, and Chadian authorities have announced free treatment for those affected.
Chad is a vast and impoverished country straddling the southern limits of the Sahara desert. Most of the country's near 10 million population live in the very south of the country where the climate and soils are more suitable to agricultural production.
There is no cure for measles, which is a highly contagious virus. Antibiotics are used for related infections - particularly of the respiratory system - that can lead to complications and ultimately death.
The most effective way to guard against measles is through vaccination, which is usually carried out in childhood.
A survey carried out by MSF in the Bousso district, 300 km south of N'djamena, found that vaccination campaigns had reached only a small proportion of the dispersed and nomadic population.
MSF in collaboration with local health authorities and the Ministry of Health launched a vaccination campaign on 21 April to immunise 40,000 children under the age of five.
"The immunisation campaign is almost complete," said Wim Fransen, MSF head of mission in Chad. "This achievement will reduce the mortality rate, protect the population and cut the spread of the epidemic."
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