Nigerian police say nine polio vaccination workers have been killed in two separate attacks on health centers in the northern city of Kano.
Witnesses said gunmen on motorbikes opened fire on the health centers early on February 8 as women waited with their children for the vaccine.
Earlier reports suggested a higher death toll.
Reports say that most, possibly all, of the victims were women.
No one has claimed responsibility but the radical Islamic sect Boko Haram is suspected of being behind the attacks.
Along with Afghanistan and Pakistan, Nigeria is one of only three countries where polio remains endemic.
At least 16 polio vaccination campaign workers were killed in Pakistan in recent months.
The Taliban has repeatedly denounced the vaccination program as a plot to sterilize people or conduct spying operations.
Based on reporting by Reuters, and the BBC