KANO, Nigeria (Reuters) - Gunmen shot dead nine health workers who were administering polio vaccinations in two separate attacks in Nigeria's main northern city of Kano on Friday, police said. Islamist militant group Boko Haram - a sect which has condemned the use of Western medicine - has been blamed for carrying out a spate of assaults on security forces in the city in recent weeks. (Reporting by Joe Brock; Editing by Janet Lawrence)