The strategy for the global campaign to eradicate polio by the end of 2005 is being changed in response to a resurgence of the paralytic disease in India, Pakistan and Nigeria, officials announced yesterday.

The campaign, now in its 15th year, will concentrate its efforts and limited money on those three countries and 10 others where the polio virus in still in circulation, or was until recently. That, in turn, will require cutting back activity in 80 countries that as recently as last year were the target of intensive polio vaccination efforts.

The number of polio cases worldwide jumped to 1,919 last year -- nearly a six-fold increase from the 2001 total of 329. India accounted for 1,556 cases, almost all in two northern states. Pakistan and Nigeria had smaller increases.

"This shift . . . entails massively shifting preventive immunization activities to key geographic areas," said Gina Tambini, a representative of the World Health Organization, at a news conference in Washington yesterday. WHO officials also held briefings in Tokyo, Geneva and Brussels to announce the change.

In India, vaccination teams have been able to reach only 80 to 85 percent of young children, even during special "immunization days," when tens of thousands of volunteers fan out to deliver polio vaccine village to village and house to house. In regions beset by poverty, crowding and poor sanitation, that leaves an unvaccinated population big enough to allow the virus to circulate indefinitely.

The eradication campaign is already five years behind schedule. The last infection was supposed to have been found no later than the end of 2000. The self-imposed deadline is now the end of 2005.

Only one human disease -- smallpox -- has ever been eradicated. The last naturally occurring case of that viral infection occurred in Somalia in 1977. (A laboratory accident in England the next year infected several other people.) One other infection, guinea worm disease (dracunculiasis), is on the path to eradication.

The main tool of the polio campaign is an oral vaccine that contains a weakened strain of the virus. It causes an infection, which in all but a few cases is harmless, that stimulates immunity against more dangerous strains. In many countries, at least three doses of vaccine are given in an infant's first year, and additional doses are given during immunization days in which every child younger than 5 is targeted.

However, polio is proving unusually difficult to eradicate. One reason is that, unlike smallpox, which causes a dramatic rash, most polio infections are invisible.

Only about 1 in 200 infected people suffers muscle weakness, paralysis or death. A single symptomatic case implies that hundreds of people nearby are carrying the virus, which is excreted in feces during the infection.

Last year, 93 countries held a total of 266 national or regional immunization days as part of the eradication effort. Under the new strategy, there will be 51 this year in 13 countries.

The countries include seven where polio is still being found -- India, Pakistan, Nigeria, Egypt, Afghanistan, Niger and Somalia -- and six at high risk of its return -- Angola, Bangladesh, Democratic Republic of the Congo, Ethiopia, Nepal and Sudan.

Routine polio immunization will continue everywhere in the world. However, 297 million doses of vaccine earmarked for regional or national immunization days in dozens of countries will now be shifted to the target regions.

The "immunization days" actually last from two to six days each, and they constitute the largest public health events in history. In January 2002, for example, 163 million children in India were given oral polio vaccine in less than a week.

The number of national and regional immunization days held in India has fallen in recent years -- from six in 2000 to three last year.

"It caught up to us when we didn't continue that intensity," said Robert A. Keegan of the U.S. Centers for Disease Control and Prevention, which helps run the campaign.

Efforts in India have been hampered in part by low vaccination rates among children of the large Muslim minority in the north. Part of the new strategy includes adding more local Muslim women to the vaccination teams and seeking the support of clerics.

Leaders of the eradication effort said the announcement doesn't mean the effort is in jeopardy.

"This is a tremendously successful initiative," said Tambini, who heads family and community health for the Pan American Health Organization, which represents WHO in the Americas.

"We're at the point where it just absolutely must come to pass. We've contributed our lifeblood to this effort," said William T. Sergeant of Rotary International. Rotary members from 30,000 clubs around the world have raised or contributed $510 million for polio eradication in the past 15 years. The service organization is nearing the end of a one-year campaign to raise an additional $80 million.

The global campaign is $275 million short of what it expects to need to finish the campaign over the next 21/2 years, the officials said.