ROBERT SIEGEL, HOST:
This week, health workers all over the world are attempting a first, to pull off the largest and quickest rollout ever of a new vaccine. It's for polio. The goal is to replace the existing vaccine with a safer one. And as NPR's Michaeleen Doucleff reports, this extraordinary effort comes with a risk.
MICHAELEEN DOUCLEFF, BYLINE: The world uses nearly 2 billion doses of polio vaccine each year. They're all stored in little vials at clinics and hospitals across the globe. Now every single vial has to be destroyed and switched out with a new one, and it all has to get done in two weeks.
WALTER ORENSTEIN: This is a tremendous amount of difficult logistics.
DOUCLEFF: That's Walter Orenstein. He's the associate director of the Emory Vaccine Center. He says countries have been training for this switch for months. Health workers have been taught to destroy the old vaccine by boiling it, incinerating it, even burying it in the ground.
ORENSTEIN: And what's being done is to go out and have independent monitors visit these sites to make sure the vaccine has been collected and destroyed.
DOUCLEFF: Do you know how many sites there are, like just scale wise?
ORENSTEIN: That I don't know, but it's huge. It's mind-boggling.
DOUCLEFF: Thousands of monitors are visiting thousands and thousands of sites. But Orenstein says it's all being done for a really good reason, to get the world closer to eradicating polio. Robin Nandy is the chief of immunization at UNICEF, which is helping with the vaccine switch out. He says the vaccine used in most countries contains a live virus. Now, the virus has been weakened, so it doesn't make people sick but...
ROBIN NANDY: In very rare instances, the live vaccine virus can mutate and cause polio.
DOUCLEFF: Last year, the world recorded about 100 cases of polio. About 30 of them were caused by mutant strains from the old vaccine. The new vaccine also has a live virus in it, but it mutates much less often. So in the long run, it should cause about 90 percent fewer cases.
But there's one big catch. You see, the new vaccine doesn't protect against one type of polio, a type that the world eradicated 15 years ago. And that's why it's so important that all those vials of the old vaccine are completely destroyed. If some aren't, some of that virus could leak out into the world, and we could have outbreaks of a type of polio we haven't seen since 1999.
NANDY: We do expect this and we have put in place measures to detect this very quickly and respond to this.
DOUCLEFF: And Nandy says it's all worth the risk because if the world is ever going to wipe out polio, we have to first make sure the vaccine isn't causing it. Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.