London ? A polio outbreak in Nigeria was caused by the vaccine designed to stop it, international health officials say, leaving at least 69 children paralyzed.

It is a frightening paradox in a part of the world that already distrusts western vaccines, making it even tougher to stamp out age-old diseases.

The outbreak was caused by the live polio virus that is used in vaccines given orally – the preferred method in developing countries because it is cheaper and doesn’t require medical training to dispense.

“This vaccine is the most effective tool we have against the virus, but it’s like fighting fire with fire,” said Olen Kew, a virologist at the U.S. Centers for Disease Control and Prevention.

The CDC and the World Health Organization announced the cause of the polio outbreak last week, even though they knew about it last year.

Outbreaks caused by the oral vaccine’s live virus have happened before. But the continuing Nigerian outbreak is the biggest ever caused by the vaccine. It also follows a nearly yearlong boycott of the vaccine in Africa’s most populous country because of unfounded fears the vaccine was a Western plot to sterilize Muslims.

Officials now worry that the latest vaccine-caused Nigerian outbreak could trigger another vaccine scare.

Experts say such outbreaks only happen when too few children are vaccinated. In northern Nigeria, only about 39 percent of children are fully protected against polio.

The oral polio vaccine contains a weakened version of polio virus. Children who have been vaccinated excrete the virus, and in unsanitary conditions it can end up in the water supply, spreading to unvaccinated children.

In rare instances, as the virus passes through unimmunized children, it can mutate into a form that is dangerous enough to spark new outbreaks.

In 2001, officials reported that 22 children were paralyzed from polio in the Dominican Republic and Haiti in this way. Subsequent vaccine-caused polio outbreaks have occurred in the Philippines, Madagascar, China and Indonesia.

In the West, the polio vaccine is given as a shot and uses an inactivated virus, but that method is more expensive and requires training.

In Nigeria, the outbreak comes “in the wake of all the other problems they’ve had,” said Dr. Donald A. Henderson, who led WHO’s smallpox eradication campaign in the 1970s.

In 2003, politicians in northern Nigeria canceled vaccination campaigns for nearly a year, claiming the vaccine was a Western plot to sterilize Muslims. That led to an explosion of polio, and the virus jumped to about two dozen countries.

Now, health officials’ decision to keep quiet about the cause of the outbreak for so long may look suspicious.

Dr. David Heymann, WHO’s top polio official, said that because the organization considered the outbreak to be a problem for scientists and not something that would change global vaccination practices, they thought it was unnecessary to immediately share publicly.