Nearly two years after radical Islamic preachers told parents to refuse to have their children vaccinated against polio for fear it was part of a U.S. plot against Muslims, the repercussions are still being felt: A Nigerian strain of the virus that causes the crippling disease has cropped up as far away as Indonesia.

The U.N. health agency says the world still has a chance to meet a deadline to stamp out polio by year's end, but other experts are pessimistic.

In Kano, northern Nigeria's largest city, many residents still refuse to have their children vaccinated, not just against polio but against other childhood diseases such as measles.

"They said the vaccines will endanger our daughters. Now they think otherwise. I am yet to be convinced," said 37-year-old father of four Mustafa Balarabe. He said his children wouldn't be vaccinated, citing "the general Western plot against Muslims worldwide."

An imam in Kano, 50-year-old Ibrahim Abubakar, was unapologetic.

"The boycott of the polio vaccine in Kano was necessary to fulfill the religious injunction, which tells us to find out about a thing when we have doubts," he said.

"I do not agree that we exported polio to any country. If these countries were carrying out vaccinations ... they should not have had any cases."

This week, Indonesia said polio had re-emerged in the country for the first time in a decade and scientists say the strain most probably came from Nigeria.

Fifteen other countries where polio had been eradicated have been re-infected from Nigeria since 2003, when northern Islamic leaders led a vaccine boycott, claiming the immunization campaigns were part of a U.S. plot to infect Muslims with AIDS or render them infertile. American officials have repeatedly said there is nothing to the allegations

Regional governors blocked U.N.-backed vaccination drives for several months, until they were satisfied in May 2004 by the purity of a vaccine, imported, ironically, from Indonesia. The preachers said supplies from a Muslim country could be trusted.

Since immunization restarted in July in Nigeria, polio has been retreating rapidly. Nigeria has seen 54 cases this year, down 40 percent from 94 a year ago, but it still accounts for close to half of cases worldwide.

While some officials with the U.N. World Health Organization say its $4 billion campaign to wipe out polio worldwide by 2005 is still on track, others are skeptical.

"I think there's a very high risk that we'll continue to have cases in 2006, especially in Nigeria and a few other African countries," said Robert Keegan of the Centers for Disease Control in Atlanta, which is a partner in the anti-polio campaign.

"We are cautiously optimistic that transmission can be stopped in 2006," he added.

Keegan said Nigeria poses the most serious risk, with its population of around 130 million, many of whom travel widely.

Since 2003, the paralyzing, waterborne illness has spread from Nigeria to Sudan, where it has infected 149 people. Along with 10 other west and central African nations, it has also spread to Ethiopia and Saudi Arabia, but vaccination campaigns have averted major outbreaks in those countries.

The spread of the disease to Yemen last month, however, has been labeled by WHO as "a major epidemic," with 22 cases confirmed in a country that had previously been considered free of polio.

Bruce Aylward, coordinator of the WHO's Global Polio Eradication Program, sees many reasons to be optimistic over the fight against polio in Nigeria.

"What's happening there, to be honest, is close to extraordinary," he said by telephone during a visit to India, one of six countries in the world where polio is still endemic.

"Here's a program that had virtually ground to a halt for around 12 months, but the leadership in the north of the country as well as in the south have responded very rapidly."

The vaccination campaigns that restarted in 2004 "began to have a bite very quickly," he said, with cases dropping rapidly even during the typical peak polio season running from November to July.

Key to this has been the support of local political, traditional and religious leaders in backing the campaign, particularly in visiting villages ahead of regular vaccination drives to persuade people of the vaccines' safety.

However, so far this year, an estimated one-fifth of all Nigerian children aged 1 to 5 targeted by the campaign have not been immunized. Although the World Health Organization says only a small proportion of this is due to continued opposition to the vaccine, it declined to give out its data.

Aylward said that while the polio campaign looked like it was on-track in five of the six endemic nations — India, Pakistan, Afghanistan, Egypt and Niger — the situation was far more uncertain in the sixth, Yemen.

Two danger countries are Sudan and Ivory Coast, where civil war and unrest have often prevented vaccination campaigns.

Polio spread from Sudan to Ethiopia this year, reappearing in border areas after four years of absence. Aylward said "the risk of explosion is very high" in Ethiopia, where the country's vast size and mountainous terrain would make a nationwide vaccination campaign difficult.

The other risk, Aylward said, is Congo, the war-beset nation bordering Sudan to the west.

These countries had already been rid of polio under previous immunization campaigns. If polio spread there "we could get into trouble and have a serious delay," Aylward said.