Political leaders and health workers launched a drive Saturday to immunise more than 80 million children against polio in 23 African nations and fight back against a resurgence of the crippling disease. Campaigners had been hoping to eradicate polio this year or next but the virus has spread in the past 18 months to 12 countries where it had been wiped out, in part because the northern Nigerian state of Kano banned the vaccination.