KHARTOUM, Sudan (AP) — Sudan has launched a massive vaccination campaign to immunize 2.4 million people against an outbreak of yellow fever in the restive region of Darfur, the U.N. said Monday.

The U.N. Office for the Coordination of Humanitarian Affairs said Khartoum’s health ministry received an initial shipment of 800,000 doses of vaccine on Friday to battle the outbreak, which began in September after a heavy rainy season created additional breeding sites for disease-carrying mosquitoes.