Effort to Defeat Polio Faces Unique Challenges

Unclear When Vaccination Can Be Halted

The troubled 19-year-old campaign to eradicate polio is celebrating recent progress and an unexpected infusion of cash, but experts are coming to realize that they will not be able to end the expensive and laborious efforts to control the virus anytime soon.

Ridding the world of polio will be a far messier business than the 1977 eradication of smallpox, which remains a unique achievement in medicine. That is because it is now clear that the virus that causes polio could reemerge years, and possibly even decades, after the last case is found.

The reason involves peculiarities of poliovirus and the oral (Sabin) vaccine being used to eradicate the disease.

The vaccine contains a weakened poliovirus that stimulates immunity against the "wild" virus, which can cause paralysis. On rare occasions, however, the vaccine virus can mutate to a more dangerous form, spread from person to person and cause a paralytic infection.

This phenomenon, only recognized in the past decade, has caused outbreaks in 10 countries since 2000. This year, 7 percent of all polio cases worldwide were caused by mutant, vaccine-derived virus.

The only way to prevent this from happening is to keep the world's infants and toddlers fully immunized -- in other words, to keep up the exhausting, expensive full-court press that has now gone on seven years longer than was anticipated.

To rid the planet of polio, people will eventually have to stop using the oral vaccine. Whether they should forgo immunization altogether or use the alternative vaccine -- the Salk "polio shot" that does not contain live virus and cannot cause infection -- is now under debate.

Switching to the Salk vaccine, however, will be hard.

It's more expensive: roughly $2.70 per dose, compared with 15 cents for the oral vaccine. About 135 million infants are born each year, and each will need at least two shots. While the two companies that make most of the world's supply can ramp up production to that level, it will take them at least five years, and possibly a lot longer.

Consequently, even after eradication, many countries will have to continue using the oral vaccine -- and be prepared to play a global version of whack-a-mole, spotting and suppressing mini-epidemics caused by it.

This all adds up to an outcome nobody anticipated in 1988 when the World Health Organization, emboldened by the smallpox success, took on polio.

"My major concern has been that the eradication of wild virus may not be the whole problem in terms of eliminating the disease, or even protecting the world from it," said Ellie Ehrenfeld, a virologist at the National Institutes of Health and an adviser to WHO.