The World Health Organization approves a new polio vaccine for emergency use

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Next, we have news of the comeback of a virus. When the global effort to eradicate polio began more than three decades ago, about 350,000 children per year were paralyzed by polio. But thanks to a robust vaccination effort, that number dropped into the dozens by 2016. Now cases have started to go back up. Here's NPR's Ari Daniel.

ARI DANIEL, BYLINE: Eliminating polio is complicated. Scientists have tried outmaneuvering the virus, but they haven't fully succeeded.

Raul Andino is a virologist at UC San Francisco.

RAUL ANDINO: Viruses rapidly evolve. They are masters in changing to overcome any kind of challenge that they encounter.

DANIEL: Andino says these genetic gymnastics are a big part of why polio has remained so elusive to defeat. In the U.S., which is polio-free, children get injected with a series of shots containing inactivated polio virus. But many other parts of the world rely on an oral vaccine made with living weakened virus. It's a one-time dose that confers lifelong immunity - quick and cheap. But there's a problem.

ANDINO: Sometimes it actually reverts and then starts to cause disease.

DANIEL: The virus reverts. It can become virulent again, even leading to paralysis in some cases - to be clear, not in the person who was vaccinated.

ANDINO: No.

DANIEL: Getting the polio vaccine does not give you polio.

ANDINO: No.

DANIEL: So the vaccine is safe.

ANDINO: Yes.

DANIEL: But those who've been immunized with the oral vaccine can shed live virus in their stool. In places with poor sanitation, the virus can spread through sewage, where it can become virulent again. And if someone who isn't vaccinated comes in contact with contaminated wastewater, they could become sick with polio, vaccine-derived polio. Gaps in immunization create more opportunities for the unvaccinated to become infected.

ANDINO: These vaccination campaigns have been certainly affected by the pandemic. And also, the focus of everybody is looking into SARS-CoV-2 rather than polio.

DANIEL: So Andino and his collaborators developed a new oral polio vaccine, one that still contains a weakened version of the virus, but that they hobbled even further.

ANDINO: What we did is three independent modifications.

DANIEL: Which Andino says makes reactivation way less likely - this new vaccine, after performing well in clinical trials, was approved for emergency use by the World Health Organization, which says 265 million doses are now being rolled out in 14 countries, primarily in Africa.

SVEA CLOSSER: This is not the first time we've heard this new vaccine is going to solve everything.

DANIEL: Svea Closser is a medical anthropologist at the Johns Hopkins Bloomberg School of Public Health who studies vaccination programs with a focus on polio.

CLOSSER: On the other hand, if you have an oral polio vaccine that truly doesn't cause vaccine-derived polio, it really could be wonderful. It's something that we'll probably need in order to secure eradication eventually. So I'm hopeful (laughter) - skeptical but hopeful.

DANIEL: Uganda is one of the countries that's embraced the new vaccine.

Sabrina Kitaka works as an infectious disease pediatrician at Makerere University in Kampala.

SABRINA KITAKA: You just need to have one child with a vaccine-derived polio infection, and before you know it, it's the rest of the continent.

DANIEL: Kitaka says there have been no vaccine-derived polio cases reported within Uganda. But the virus was detected in wastewater. And officials worry about refugees coming from conflict zones in neighboring countries where vaccination programs have lapsed. Kitaka says in just two weeks in January, Uganda vaccinated more than 8 million children with the new vaccine with no side effects reported.

KITAKA: For us as pediatricians, as researchers - we think that this is going to be part of the endgame to end polio in the whole world. I am optimistic, but I'm not yet comfortable that we are out of the woods.

DANIEL: Ari Daniel, NPR News.

(SOUNDBITE OF ARDEN'S "EDEN") Transcript provided by NPR, Copyright NPR.