Filed at 3:13 a.m. ET

WASHINGTON (AP) -- When a vaccine containing a live virus collided with a population that was poorly immunized, an outbreak of polio resulted among children in Haiti and the Dominican Republic, according to a new study.

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

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 Prevention 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 that the outbreak resulted from unprotected children coming in contact with children who had received the oral vaccine.

Patients given the oral 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, said Kew.

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

He said that there was a false sense of security in Haiti because natural polio had been wiped out. As a result, the 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 in the general population is very high, about 90 percent.

Kew said that in Haiti and the Dominican Republic, after the introduced polio virus infected some in the 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 about the disease, said Kew.

``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 rare 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.

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, said Kew, 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 outbreaks from the oral polio vaccine are unlikely.

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