ARI SHAPIRO, HOST:

The first wave of a massive polio vaccination campaign is wrapping up in Gaza. The World Health Organization says, in less than two weeks, they've helped administer more than 550,000 polio vaccines to children across the enclave. But the work is, at best, half done. Health officials must soon give kids a second dose. Science reporter Ari Daniel has more.

ARI DANIEL, BYLINE: Dr. Majdi Duhair is an epidemiologist from Gaza who worked for the territory's Ministry of Health for 30 years. Today, when he looks around this place he calls home, he sees wreckage and devastation.

MAJDI DUHAIR: No clean water - drinking water, no hygiene.

DANIEL: Which has led to plenty of sewage in the streets. This is the perfect recipe for the transmission of diseases, including polio, a highly infectious virus that can spread through fecal matter. So when polio turned up in wastewater samples in the enclave, alarm bells went off, especially because vaccination programs in Gaza have collapsed since the war began. Sure enough, last month, a little boy, not quite 1 year old, became paralyzed in both of his legs. He's Gaza's first case of polio in a quarter century.

DUHAIR: On confirmed case of polio - it is considered as an outbreak.

DANIEL: An outbreak because a single case points to roughly 200 more people who are infected but not showing symptoms. No other cases have surfaced in Gaza so far, but...

DUHAIR: We are expecting the circulation of this virus is very high (ph).

DANIEL: So public health officials moved quickly to launch a massive campaign of oral polio vaccination. The goal - to reach at least 90% of the kids in Gaza under 10. The big challenge was how to do it in the middle of a war zone. Juliette Touma is with the U.N. Relief and Works Agency, the group that's doing the actual vaccinating.

JULIETTE TOUMA: It's impossible to undertake a vaccination campaign of such volume and scale under a sky full of airstrikes. Impossible.

DANIEL: That's why it was crucial that the Israeli army and Hamas agree to briefly pause the fighting in different areas. Usually, a campaign like this would go house to house to vaccinate as many kids as possible. But in Gaza, they couldn't, since there are so few houses left, and people are often moving hour by hour.

TOUMA: We go from shelter to shelter and tent to tent. People are literally everywhere. I mean, in the middle of the street. You have people living by the beach. So the idea is to reach every child wherever they are.

DANIEL: The World Health Organization says that families were enthusiastic about having their children vaccinated. Richard Peeperkorn is with the global health body.

RICHARD PEEPERKORN: We are happy with this polio campaign. And I think we are also quite confident that we reached an enormous amount of children in this short time.

DANIEL: In the coming weeks, the whole undertaking will happen again as the teams try to track down the same more than half a million children and give them their second dose of the vaccine.

SCOTT ANDERSON: I don't think a polio campaign has ever been done like this globally, constrained by time and security as we're currently undergoing in Gaza.

DANIEL: Scott Anderson is deputy humanitarian coordinator with the U.N. He's stationed in Gaza, and he likens this vaccination effort to putting a band aid on a much deeper wound, a territory whose people and infrastructure have been bombarded for over 11 months.

ANDERSON: We're really just hoping and praying that there's not a larger outbreak of disease.

DANIEL: Vaccination is just a piece of the polio response. For the next six months, officials will need to be vigilant to ensure there's no virus circulating in neither the people nor the wastewater of Gaza. And that's when they'll know if the outbreak is over. For NPR News, I'm Ari Daniel.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.