Instability and insecurity in some West and Central African nations are threatening the success of a 20-country polio vaccination campaign, which aims to immunize 111.1 million children against the disease, IRIN reports. Ongoing insurgent attacks threaten the campaign in Nigeria, the region’s only polio-endemic country and home to 57.7 million of the children targeted, the news service notes. Parts of Mali, Niger, and Chad also pose security problems for health care workers trying to access children in remote or disputed areas, according to IRIN. “Human error and weak health systems also play an important role in sub-optimal immunization reach,” the news service writes, noting so far, “only Ghana, Cape Verde, Burkina Faso, Gambia, and Togo have achieved the required 90 percent coverage, according to UNICEF” (3/23).