Sixty-nine children in northern Nigeria contracted polio following a vaccination against the disease, a WHO official in Nigeria said on Thursday.
"They were vulnerable (to this type of virus) which they hadn't been vaccinated enough. These are extremely rare cases, however," WHO representative Peter Eriki told AFP.
He ruled out that the virus could have come from contamination of the vaccine. In rare cases, the virus in the vaccine can mutate into a form that can paralyse, according to WHO.
"Those who are vaccinated are completely protected. The better thing to do is to ensure that all children are vaccinated," he added.
The vaccination campaigns launched by the United Nations and the Nigerian government came up against resistance from Muslim chiefs in the northern part of the country in 2003. Some argued that the polio vaccination was part of a plot hatched by the US to sterilise Muslims.