The death toll from a meningitis outbreak in Nigeria has passed 1,000 as a vaccination program struggles to keep up with demand in Africa’s most populous country.

Nigeria had recorded a total of 13,420 suspected cases as of May 8, with 1,069 deaths, the Nigeria Center for Disease Control (NCDC) said in its latest update.

The outbreak began in the northern state of Zamfara in December 2016 and has spread rapidly across northern Nigeria. The outbreak is Nigeria’s biggest since a 1996 meningitis epidemic, when more than 100,000 cases were reported and 11,700 deaths recorded.

Nigerian health officials launched a mass vaccination campaign in April, aiming to inoculate half a million people. But the country is relying on additional deliveries of vaccine in order to meet the demands of its population of more than 180 million. The NCDC statement said that a delivery of almost 700,000 doses of the vaccine was due soon.

The disease has spread to 23 of the country’s 36 states but does appear to be slowing down. The NCDC statement recorded that the worst-affected states of Zamfara, Sokoto, Katsina and Kebbi have all seen a drop in the number of cases, while two states—Kebbi and Niger—have not had any deaths.

Nigeria meningitis baby A woman pacifies her daughter, who is suffering from brain damage resulting from meningitis at Molai General Hospital in Maiduguri, Nigeria, on November 30, 2016. More than 13,000 people have been affected by a meningitis outbreak in Africa’s most populous country. Afolabi Sotunde/Reuters

It has been caused by a strain of type C meningitis, which is not common in Nigeria—previous outbreaks were caused by type A, a different variant of the bacteria that causes the disease. The worst-affected group has been children aged between five and 14 years old, according to the BBC.

Meningitis is a bacterial infection that causes inflammation of the membrane surrounding the brain and spinal cord. It is spread through close contact, including kissing or coughing on someone. Symptoms include a stiff neck, high fever, headaches and vomiting. The disease can cause serious brain damage and between 5-10 percent of patients die within 48 hours of the onset of symptoms, according to the World Health Organization.

Nigeria lies on the so-called meningitis belt, which stretches across sub-Saharan Africa from Senegal in the west to Ethiopia in the east. The region has the highest rates of the disease in the world.