U.N. agencies say they are getting ready to launch the largest-ever mass immunization campaign to fight an outbreak of polio in India. The agencies are planning to vaccinate 165 million children.

Officials at the U.N. children's fund and the World Health Organization say a huge polio epidemic is raging in the state of Uttar Pradesh in northern India.

A World Health Organization expert on vaccines, Dr. Daniel Tarantola, says U.N. agencies and Indian authorities are moving quickly to vaccinate children in every house in Uttar Pradesh to try to prevent the further spread of polio. "The problem of this epidemic in Uttar Pradesh is that it not only affects the population in Uttar Pradesh," he said. "But it represents a threat as well to other states in India, some of which were able to get rid of poliomyelitis, were free of poliomyelitis for several months and are now being re-infected with a sort of infection of cases from Uttar Pradesh. This would be the case of Gujarat and West Bengal, for example. And there is a worry about neighboring Bangladesh."

Dr. Tarantola says if 95 percent of Uttar Pradesh's children are vaccinated, the polio virus will be halted.

Carl Tintsman is a polio expert with the U.N. children's agency. He says if the anti-polio campaign is to be effective health workers have the daunting task of reaching millions of young people in a very short period of time. "The challenge of trying to reach all 165 million children one-by-one-by-one in a period of six days is huge," said Mr. Tintsman. "It has never, ever been done before. And part of the problem with our earlier rounds in India is while we did reach 95 percent or more of the children in most states of India, we did not succeed in doing that in all of Uttar Pradesh."

U.N. health officials say the success of the campaign in Uttar Pradesh will go a long way in determining whether a global initiative aimed at eradicating polio worldwide by the year 2005 will succeed.