The international team that conducted the study of the outbreak, appearing online in Science this week, said it points to the need for complete vaccination coverage.

The oral polio vaccine used in the two countries contained a weakened version of the polio virus.

An inoculated person would be exposed to the virus, and have immunity to future infection, without experiencing symptoms.

The vaccine virus was somehow released into the wild, probably shed by someone who was vaccinated in 1998 or 1999.

The weakened virus, released in the environment, interacted with a naturally-occurring virus related to polio called an enterovirus.

The combination of those two resulted in a fully virulent polio virus that caused paralysis and could be transmitted from one person to another.

In fact, the scientists found the virus that was derived from the vaccine was indistinguishable from a natural polio virus.

Most of the 21 polio cases in the outbreak, including 2 deaths, involved children who were unvaccinated or incompletely vaccinated against polio.

Similar outbreaks, originating from a vaccine virus, occurred in the Philippines and Egypt, both poorly immunized regions, the authors said.

Written by CBC News Online staff