The failure to eradicate polio in India and a few other countries has stalled international efforts to eliminate the paralyzing childhood disease globally. Now, a new study explains why the virus persists in India, despite massive immunization efforts.

An Indian health worker gives oral polio vaccine to a child in India

Polio was supposed to have been erased everywhere six years ago. 2000 was the target year set by governments at the World Health Assembly in 1988, a time when the virus was paralyzing more than 1,000 children a day.

But six years past the 2000 target, polio remains unconquered. Some experts have suggested that eradication may not be achievable and that control might be a more realistic goal.

To be sure, much progress has been made. Most of the world is polio-free. But an expert on infectious disease distribution at Imperial College London, Nicholas Grassly, points out that the virus remains tenacious in India and three other countries.

"In 2005, there were less than 2,000d cases of paralysis for the whole year and all but four countries had stopped polio transmission at some point," he said.  "Those remaining four countries - India, Nigeria, Pakistan, and Afghanistan - have led to exports of infection and outbreaks elsewhere. So it is of key importance that transmission is stopped in those four countries."

Grassly says distribution problems have caused continuing polio transmission in Nigeria, Pakistan, and Afghanistan, but that has not been the case in India, the world's second most populous country. Children there have received many more doses of vaccine than in other endemic countries.

"In India, there is particular concern because polio virus continues to spread despite the implementation of a very large vaccination program that is reaching most of the children there," he added.

Grassly and his colleagues explain the failure in a study published in the journal Science. They performed a computer analysis of reports of paralysis in Indian children and determined the conditions that influence polio's persistence.

The greatest problem is in the poor northern states Uttar Pradesh and Bihar, the source of India's recent polio outbreaks. The analysis says high population density and poor sanitation there are the key obstacles to eradicating the virus because they make it easier for polio to spread and they decrease the power of the vaccine.

"In these parts of India, children are often infected with other diseases, particularly diarrhea," Grassly noted.  "In these children, the oral vaccine does not work, does not work so well. Children don't respond to the vaccine. It doesn't stay in their gut for long enough. So while the vaccine has worked very well in the rest of India, in these particularly difficult conditions, it works less well."

The vaccine that had been in use in India contains weakened versions of three polio strains to produce immunity, a so-called trivalent vaccine.  But Grassly says the strains can interfere with each other and cause less than desirable immunity in a child whose health is already compromised with other illnesses. Since only one of the strains is prevalent in India, he says the government's switch last year to a monovalent formula containing only that strain should boost the vaccine's effectiveness.

"It doesn't suffer from competition between the three vaccine strains that exist in the trivalent vaccine," he added.  "So by cutting out that competition, efficacy is typically two or three times higher than the trivalent vaccine. It raises hopes that polio could soon be eliminated from India and, in fact, globally."

India has set a 2007 deadline for this country-wide goal, but there is another obstacle to it that Grassly's work does not address. News reports say some parents in Uttar Pradesh are alarmed at a recent story in an Urdu language newspaper that the vaccine could cause cancer and have refused to inoculate their children.