The dangers of only partially vaccinating a population against polio was underscored today in a report that blamed an outbreak of the deadly disease in the Caribbean on incomplete vaccination, which allowed the virus to mutate, escape and then infect 21 people, killing two children.

"The outbreak probably began in Haiti, when a routine oral poliovirus vaccine dose was given to a child living in a community with low vaccine coverage," the researchers, led by Olen Kew of the U.S. Centers for Disease Control and Prevention, wrote today in the journal Science.

One of those who died was a 12-year-old boy and another was a 35-month-old boy. "He didn't see his third birthday," Kew said. "If they had only used the vaccine appropriately, this would not have happened."

Two factors are to blame for the outbreak, the researchers say. One is the low vaccination rate in Haiti and the Dominican Republic, which share an island in the Caribbean.

In Haiti, political strife and poverty have interfered with vaccination efforts, Kew said.

The second factor was the use of an oral vaccine, which uses a live but weakened strain of the virus. The oral vaccine, developed by Albert Sabin, helped rid many countries of polio, including the United States, because it is easy to administer and highly effective. But the virus used is still alive, although it has been altered so it should not cause disease.

It stays alive in the body, and it turns out, can hook up with other, related viruses called enteroviruses. They swap DNA and the polio virus can become deadly and highly infectious again. It is shed in the feces and gets into the water supply.

People get polio from infected water, so when sewage gets into drinking or washing supplies, the newly energized polio virus can infect them with crippling effects.

Kew said the study shows the importance of getting everyone vaccinated, as the few vaccinated children were, in essence, infecting everyone else.

"Even in developed countries, when people don't vaccinate their kids, the virus can find them," Kew said.

Most people infected with polio have no symptoms. But in a small percentage of cases, the virus attacks the nerves -- usually the spinal cord -- and can paralyze the victim. Between 2% and 5% of children with paralytic polio die and up to 30% of adults.

Humans are the only animals that can be infected with polio, so health officials think vaccination can wipe it out.

There is another vaccine, the inactivated poliovirus vaccine based on one invented by Jonas Salk. This injected vaccination is now widely used because the virus is killed and cannot become dangerous again. But people who receive the injected vaccine can become infected with wild polio and, while they do not become sick, can spread it.

Kew and colleagues noted that outbreaks of polio have been traced to the oral vaccine not only in Haiti and the Dominican Republic, but in the Philippines and Egypt. But they stressed that the oral vaccine is still useful, not least because it works better.

"We don't want people to jump to the conclusion that inactivated polio vaccine [the injected version] is the answer," Kew said.

The World Health Organization wants to eradicate polio globally, but says the effort has been hampered by a lack of funds and political will in the affected countries. In the United States, polio killed thousands of people and disabled tens of thousands before it was wiped out in 1979.