The killings, with eerie echoes of attacks that killed nine female polio workers in Pakistan in December, represented another serious setback for the global effort to eradicate polio.
Most of the victims were women and were shot in the back of the head, local reports said.
A four-day vaccination drive had just ended in Kano State, where the killings took place, and the vaccinators were in a “mop-up” phase, looking for children who had been missed, said Sarah Crowe, a spokeswoman for the United Nations Children’s Fund, one of the agencies running the eradication campaign.
Dr. Mohammad Ali Pate, Nigeria’s minister of state for health, said in a telephone interview that it was not entirely clear whether the gunmen were specifically targeting polio workers or just attacking the health centers where vaccinators happened to be gathering early in the morning. “Health workers are soft targets,” he said.
No one immediately took responsibility, but suspicion fell on Boko Haram, a militant Islamist group that has attacked police stations, government offices and even a religious leader’s convoy.
Polio, which once paralyzed millions of children, is now down to fewer than 1,000 known cases around the world, and is endemic in only three countries: Nigeria, Pakistan and Afghanistan.
Since September — when a new polio operations center was opened in the capital and Nigeria’s president, Goodluck Jonathan, appointed a special adviser for polio — the country had been improving, said Dr. Bruce Aylward, chief of polio eradication for the World Health Organization. There have been no new cases since Dec. 3.
While vaccinators have not previously been killed in the country, there is a long history of Nigerian Muslims shunning the vaccine.
Ten years ago, immunization was suspended for 11 months as local governors waited for local scientists to investigate rumors that it caused AIDS or was a Western plot to sterilize Muslim girls. That hiatus let cases spread across Africa. The Nigerian strain of the virus even reached Saudi Arabia when a Nigerian child living in hills outside Mecca was paralyzed.
Heidi Larson, an anthropologist at the London School of Hygiene and Tropical Medicine who tracks vaccine issues, said the newest killings “are kind of mimicking what’s going on in Pakistan, and I feel it’s very much prompted by that.”
In a roundabout way, the C.I.A. has been blamed for the Pakistan killings. In its effort to track Osama bin Laden, the agency paid a Pakistani doctor to seek entry to Bin Laden’s compound on the pretext of vaccinating the children — presumably to get DNA samples as evidence that it was the right family. That enraged some Taliban factions in Pakistan, which outlawed vaccination in their areas and threatened vaccinators.
Nigerian police officials said the first shootings were of eight workers early in the morning at a clinic in the Tarauni neighborhood of Kano, the state capital; two or three died. A survivor said the two gunmen then set fire to a curtain, locked the doors and left.
“We summoned our courage and broke the door because we realized they wanted to burn us alive,” the survivor said from her bed at Aminu Kano Teaching Hospital.
About an hour later, six men on three-wheeled motorcycles stormed a clinic in the Haye neighborhood, a few miles away. They killed seven women waiting to collect vaccine.
Ten years ago, Dr. Larson said, she joined a door-to-door vaccination drive in northern Nigeria as a Unicef communications officer, “and even then we were trying to calm rumors that the C.I.A. was involved,” she said. The Iraq and Afghanistan wars had convinced poor Muslims in many countries that Americans hated them, and some believed the American-made vaccine was a plot by Western drug companies and intelligence agencies.
Since the vaccine ruse in Pakistan, she said, “Frankly, now, I can’t go to them and say, ‘The C.I.A. isn’t involved.’ ”
Dr. Pate said the attack would not stop the newly reinvigorated eradication drive, adding, “This isn’t going to deter us from getting everyone vaccinated to save the lives of our children.”
Aminu Abubakar contributed reported from Kano, Nigeria.
