A mutated strain of polio virus traced to the oral polio vaccine has infected people in the Dominican Republic and Haiti, causing the first cases of the disease in the Western Hemisphere since 1991. None of the patients with polio has died.

"This is not a desperate situation from the view of outbreak control ... but it is still a surprise," said Pan American Health Organization spokesman Daniel Epstein on Sunday.

The vaccine, known as Sabin 1 oral poliovirus vaccine, uses a weakened version of the virus.

Although none of the patients had received the vaccine, doctors believe they caught the mutated strain from someone else who had been vaccinated. That person likely received the standard vaccination that mutated within them.

Scientists traced the cases back to the vaccine by an analysis of the mutated polio strain, which shares 97 percent of its makeup with the one used in the vaccine.

The last case of polio in the Americas was diagnosed in Peru in 1991.

The only other previously known case of an oral vaccine mutating into a virulent strain was in Egypt between 1983 and 1993, where more than 30 people were infected.

Associated Press Online, December 3, 2000