KANO, Nigeria — Gunmen suspected of belonging to a radical Islamist sect shot and killed at least nine women who were taking part in a polio vaccination drive in northern Nigeria on Friday, highlighting the religious tensions surrounding the inoculation of children in one of the few nations where the disease still remains endemic.

The attack shocked residents of Kano, the largest city in Nigeria’s predominantly Muslim north, where women often go from house to house to carry out the vaccination drives as Muslim families feel more comfortable allowing them inside their homes than men. It also signaled a new wave of anger targeting immunization drives in Nigeria, where clerics once claimed the vaccines were part of a Western plot to sterilize young girls.