From Polio in India: Many steps up… and a long one back :
"There are two types of polio vaccine: injectable or IPV, which uses a killed virus, and oral or OPV, which uses a weakened live virus. (For those into mid-20th century history, IPV is the Jonas Salk vaccine — announced to enormous fanfare in 1955 — and OPV is the Albert Sabin vaccine.) Industrialized countries overwhelmingly use IPV. But developing countries, and the global eradication campaign, use OPV, for several reasons. It’s cheaper to produce and especially to deliver, since it is administered in a couple of drops of liquid — which means no syringes to buy or dispose of, and no need for trained health-care personnel to give it. But it also confers protection not only on the kids who receive it, but on other kids nearby — because the weakened virus replicates in the gut, is shed in feces, and therefore can indirectly inoculate any child who accidentally swallows fecal particles from the vaccinated one.
But if the vaccine virus sheds its attenuating mutations and reverts to the infectious strength of the wild-type viruses, it can spread through that same route. It then poses a risk to anyone who is not currently vaccinated. As poorer countries become polio-free, they often drop vaccination as an unnecessary expense, leaving any children born after the end of vaccination vulnerable to a VDPV infection.
If you do the rough math in your head, you might be thinking, Well, this hypothetical country is polio-free already, so where’s the full-strength VDPV coming from? The answer lies in a further complexity of the vaccine virus: In certain people with certain immune deficiencies, it not only reverts, it replicates — and is shed — for a very long time, months to years. The immune deficiency transforms the vaccinated child into a polio version of Typhoid Mary, and into a severe risk of unpredictable duration.
VDPV is an unintended consequence of success: It only becomes a risk when a country gets so close to being polio-free that there is little or no wild virus to compete with the vaccine-derived one. But repeated outbreaks of VDPV have already brought polio roaring back to places that thought they were done with the disease. VDPV brought polio back to the Americas, via an outbreak in Hispaniola and the Dominican Republic, in 2000. It was also imported into the United States in 2005 via a woman who had chosen not to be vaccinated and who picked up the vaccine-derived virus from the recently vaccinated child of a Costa Rican host family.
Most famously in public heath circles, VDPV caused a huge, ongoing outbreak in Nigeria, starting in 2006, that has racked up almost 300 cases so far — straining the fragile acceptance of polio vaccine that had only just been restored after a number of imams in the north of Nigeria persuaded their congregations in 2003 to turn away from the vaccination campaigns. (For the worldwide accounting of VDPV outbreaks, see this World Health Organization table.)
VDPV will remain a risk as long as OPV is used, because it’s OPV itself that creates the risk. The only way to put an end to that risk, forever, is to conclude the polio campaign by deploying IPV in all the countries that have been using OPV. That will force the eradication campaign to take on at least some of the expense that it had hoped to reduce by using OPV instead. And that’s the true significance of the Indian girl’s case last week: the “ticking time bomb” is not just the threat of additional polio, but the threat of a huge additional bill for a campaign that is constantly on the verge of donor fatigue."
(via Not Exactly Rocket Science )
See also Vaccine-related polio a ticking time bomb in S India from Deccan Herald.