Filed at 5:57 p.m. ET

WASHINGTON (AP) -- An outbreak of polio in Haiti and the Dominican Republic has been traced to a weakened virus in a vaccine that mutated and spread disease among poorly immunized children.

Even though Haiti and the Dominican Republic were declared polio-free in the 1980s, both countries reported cases of the paralyzing disease in the summer of 2000. The countries share the Caribbean island of Hispaniola.

Thirteen children were infected in the Dominican Republic and there were eight cases, including two deaths, in Haiti.

Teams from the Centers for Disease Control and Prevent and the Pan American Health Organization quickly responded, with disease detectives looking for the source of the disease.

In a study appearing Friday in the electronic version of the journal Science, researchers report that they found the new cases of polio originated from the modified polio virus that was used in the oral vaccine, which is usually given on sugar cubes.

Olen M. Kew of the CDC, first author of the study, said the outbreak resulted from unprotected children coming in contact with children who had received the oral vaccine.

Patients given the vaccine develop a mild form of polio that results in an immunity to the disease. When this happens in a population that has not been inoculated, there is a chance that others may get the disease, Kew said.

``The virus normally does not spread from person to person if the community has a high vaccine coverage,'' Kew said. ``In Haiti, after the wild-type polio virus had been eradicated, the only source was through the polio vaccine.''

He said there was a false sense of security in Haiti because natural polio had been wiped out. As a result, immunization efforts were relaxed. In some villages, the rate of polio vaccination had been allowed to drop to about 7 percent. That primed the children for an outbreak of from the oral vaccine.

In the United States, oral polio vaccine causes only rare, isolated cases of polio because immunization against the disease is very high, about 90 percent.

Kew said that in Haiti and the Dominican Republic, after the introduced polio virus infected a poorly immunized population, it mutated and became even more virulent.

The outbreak was quickly ended with major immunization drive that pinched off the polio spread and protected the population, but the experience taught public health officials a lesson, Kew said.

``We've got a wake up call that said you've got to maintain a high level of immunization in order to be safe,'' he said.

Concerns about giving some patients polio led the CDC, the American Academy of Family Physicians, the American Academy of Pediatricians and others to recommend that children be treated with injectable polio vaccines made with killed virus instead of the oral vaccine made with the modified live virus.

Dr. Thomas N. Saari, a professor of pediatrics at the University of Wisconsin School of Medicine, said that the Hispaniola study ``points to the wisdom'' of switching to the injectable polio vaccine.

``Rare individuals who took the oral vaccine can harbor the strain over a long period of time, something that doesn't happen with the injectable,'' he said.

Saari, who serves on a committee at the American Academy of Pediatrics, said the study also showed the importance of a nation maintaining a high level of immunization.

``The main thing that prompted the outbreak (in Hispaniola) was the low rate of coverage,'' he said.

Kew said that in the developing world the oral vaccine still is used because of the lower cost and the scarcity of trained personnel required to distribute injected vaccines.

He said as part of an international effort to eradicate polio, medical teams have conducted massive oral polio vaccination drives that immunize a whole population in a community within a short time. For instance, Kew said, 100 million people were immunized in India in one day -- and the effort was repeated a month later.

When so many people are inoculated at once, said Kew, the chances of polio outbreaks are unlikely.

Dr. Neal A. Halsey of the Johns Hopkins Bloomberg School of Public Health said the experience in Hispaniola shows that developed countries need to vigorously assist poor countries in maintaining high levels of vaccination and disease control.

``We can't ignore these problems,'' he said, ``because what happens there can pose a risk to the rest of the world.''

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