All schools in and around Niger's capital, Niamey, have been shut until Monday because of a meningitis outbreak that has killed 85 people this year.
A shortage of vaccines to treat the current strain has caused the outbreak to spread, the authorities say.
A campaign to vaccinate all children between two and 15 will begin on Friday, but only half of the 1.2m doses needed are currently available.
The prime minister has asked for help getting the remaining doses.
"For the rest of the needed vaccines, we are appealing to all our partners to come to our aid to supplement the stock of vaccine doses we direly need," Reuters news agency reports Brigi Rafini as saying on national television.
The authorities have warned people against using unauthorised vaccines, saying the doses might be for the wrong strain of the disease, the agency says.
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The health minister says 905 cases have been recorded in seven of the country's eight regions, but mostly in Niamey and Dosso where it is now endemic.
"We have decided to close down schools in the region of Niamey because we have to understand why this strain has become epidemic," Health Minister Mano Aghali told the BBC French service.
"What makes the situation worse is that the 'w' strain is now epidemic and that was not the case before," he said.
Meningitis is an infection of the meninges - the membrane that surrounds the brain and spinal cord.
Symptoms can include severe headaches, fever, drowsiness, stiff neck, vomiting, confusion and a fear of light. A rash may also appear.
According to the World Health Organization, it can cause severe brain damage and is fatal in 50% of cases if untreated.
There is a "meningitis belt" in sub-Saharan Africa, stretching from Senegal to Ethiopia, with high rates of the disease, the WHO says.