The goal of eradicating polio from the world by next year is looking increasingly distant, partly because some states in northern Nigeria have at some points refused to take part in vaccination campaigns.
Some Muslim leaders say the vaccine is part of a United States plot to make Muslim women infertile. BBC News Online looks at the background to the polio campaign.
Is the polio vaccine safe?
The World Health Organisation says so, and several studies have confirmed this.
But one test did find traces of the reproductive hormone oestrogen in the vaccine, fuelling the fears.
A new governemtn study was carried out and concluded that the vaccine was safe, but in the meantime, the immunisation drive was suspended in the two northern Nigerian states of Kano and Zamfara.
With the polio drive resuming, Kano is still saying it is not convinced the vaccine is safe and is seeking supplies from Asian countries
Where did the fears come from?
Many northern Nigerians have been deeply suspicious about all vaccinations for years.
Some radical Muslim preachers say that they are unIslamic - if God wants you to die, you will; if he doesn't, you won't.
Such fears were fuelled in 1996, when United States drugs company Pfizer used an untested vaccine against bacterial meningitis in Kano.
Local people say that 11 children died as a result and sued Pfizer.
Pfizer denied the charges, saying the study was properly carried out.
It said it had received the approval of both the Nigerian government and the families of the treated patients.
In 2001, fears resurfaced over a meningitis vaccine, with reports that it contained HIV and could cause sterility.
And all the attention being given to the drive to eradicate polio from the area has only made some people even more suspicious.
"Why polio, why not malaria or any other disease?" they ask.
How has the bid to eradicate polio been affected?
Worldwide, it has been extremely successful.
In 1988, there were 350,000 cases of polio - last year just 700 were reported.
But this was up from 483 in 2001.
Almost half of the world's new cases are in Nigeria, mostly in the north.
The disease is now spreading to several neighbouring countries, where it had been thought that polio had been eliminated.
What is polio?
The disease, which once affected millions of children, attacks the central nervous system, often causing paralysis, muscular atrophy and deformity.
Between 5% and 10% of those infected die when their breathing muscles become paralyzed.