There's a heated battle going on about the Summer Olympics in Rio de Janeiro. Nearly 200 scientists signed a letter to the World Health Organization last week, calling for the games to be moved because of the ongoing epidemic of Zika in Brazil.
But many health officials — including those at WHO — say having the games in Rio doesn't pose a big enough threat to warrant moving them.
So who's right?
To figure out whether Zika might be a big problem at the Olympics, there's one key piece of information you need: How many mosquitoes will be in Rio during the games?
That's exactly what epidemiologist Mikkel Quam has been working on. He used a mathematical model — and data from another outbreak in Rio — to estimate the chance spectators and athletes will get a mosquito bite for three weeks in August, when the games take place.
"I was legitimately surprised," says Quam, who works at Umea University in Sweden. "There's very little mosquito activity during the Olympics."
August is winter in Brazil. It's cooler and drier. So the mosquito population is way down.
Only about 4 percent of fans will get bitten at least once by a mosquito that could carry Zika, Quam estimates. The chance they'll catch Zika is even lower — much, much lower.
"I think we'll get cases but I don't expect many cases," Quam says.
It's hard to calculate the exact number. But a preliminary model suggests that, at most, 1 in 31,000 people at the games will get infected with Zika, Quam and his colleagues recently reported.
Officials are expecting around 500,000 spectators and athletes. Then the model predicts, there will be — at most — 16 cases of Zika at the Olympics.
So attendees are much more likely to get the flu or food poisoning at the games than Zika, the European Center for Disease Prevention and Control concluded.
"If I would be an athlete competing, from what I've read, I would be more concerned about the pollution in the water than Zika," says Alessandro Vespignani, at Northeastern University in Boston.
But whether the Rio games pose a danger to the world isn't just about the number of Zika cases, Vespignani says. It's also about where those cases go — what's the chance a fan or athlete brings the virus home to a place without Zika and triggers a new outbreak in Africa or Asia — or here in the States?
So Vespignani is working on a computer model for the U.S. government to predict how Zika will spread. Keeping the games in Rio doesn't seem to change the course of the epidemic in his models.
"There are already so many cases around the world that adding a little bit more cases is not going to make a difference at this point," Vespignani says.
So there's no reason to move the games because of Zika, he believes.
So far the continental U.S. has had about 600 cases of Zika. They've all come from travelers to other countries. Each year hundreds of millions of Americans travel to countries where Zika is circulating, a recent study found.
"The Olympics would represent less than 0.25 percent of all travel to Zika-affected areas," Dr. Thomas Frieden, the Centers for Disease Control and Prevention's director, said last week to reporters.
So even if the Olympics were called off, "we'd still be left with 99.75 percent of the risk of Zika continuing to spread," Frieden said.
But all these predictions and models are just that — predictions. Like weather predictions, they are often wrong. And they're based on many assumptions, such as the idea that Zika behaves similarly to the way other mosquito-borne viruses do.
"The problem is, we just don't know that," says Arthur Caplan, a bioethicist at New York University. He's one of the researchers that wrote the letter to WHO, calling for the Olympics to be moved. He says the stakes are too high.
"I think it's ethically dubious to run the Olympics when you've got an epidemic of a virus that we don't understand very well," Caplan says.
For instance, scientists still don't know how long Zika can linger in the body or how big of a problem sexual transmission is.
So Caplan says, why not not err on the side of caution — especially considering the devastating effects Zika can have for mothers and their babies.
KELLY MCEVERS, HOST:
There is a debate going on about this summer's Olympic Games. Nearly 200 scientists have signed a letter to the World Health Organization calling for the games to be moved out of Rio de Janeiro. They're worried visitors to the Olympics will catch the Zika virus and take it back home and cause new outbreaks. The WHO has resisted their calls. NPR's Michaeleen Doucleff looks at the science to evaluate the risks.
MICHAELEEN DOUCLEFF, BYLINE: One researcher who wants the Olympics moved is Arthur Caplan, a bioethicist at New York University. He helped write the letter to WHO. The WHO disagreed, saying there was no health reason to move the Olympics. But Kaplan says the stakes are just too high.
ARTHUR CAPLAN: I think it's ethically dubious to run the Olympics when you've got a epidemic with a virus that we don't understand very well. There's just a lot that, apparently, we don't know.
DOUCLEFF: Like how long Zika can linger in the body or how big of a problem sexual transmission is. Scientists don't know that much about Zika. It only emerged as a problem a few years ago. But it can cause devastating birth defects, so Kaplan says, err on the side of caution. But here's another thing - Zika is spread by mosquitoes, and scientists do know a lot about how that works. They can even predict how many mosquitoes will be in Rio during the games. That's exactly what biologist Mikkel Quam has been working on at Umea University in Sweden. He built a computer model to estimate the chance fans and athletes will get bitten by a mosquito. I talked to him on Skype, and he says his results were much lower than he expected.
MIKKEL QUAM: I was legitimately surprised. There's very little mosquito activity during the Olympics.
DOUCLEFF: Quam found that only 4 percent of people at the Olympic Games will get a mosquito bite. The chance they'll get bitten by a mosquito carrying Zika is even lower - much, much lower.
QUAM: I think we could get cases, but I don't expect many cases.
DOUCLEFF: Quam says it's hard to calculate the exact number, but a preliminary model suggests, at most, 16 people will get infected with Zika during the Olympic Games - 16 out of the half-a-million attendees. Alessandro Vespignani at Northeastern University agrees with Quam's prediction. He says fans and athletes are much more likely to get the flu and food poisoning than Zika.
ALESSANDRO VESPIGNANI: You know, if I would be an athlete competing, from what I've read, I would be more worried about the pollution in the water than Zika.
DOUCLEFF: But, Vespignani says, whether the games pose a danger to the world isn't just about the number of Zika cases. It's also about where those cases go. What's the chance a fan brings the virus home and triggers a new outbreak in another country? So Vespignani is working on a computer model for the U.S. government to predict how Zika will spread both here and around the world. He added the extra travelers from the Olympics and ran the models again. What he saw was reassuring. Keeping the games in Rio doesn't seem to change the course of the epidemic.
VESPIGNANI: There are already so many cases around the world, you know, that adding a little bit more of cases is not making a difference at this point.
DOUCLEFF: So there's no reason to move the games because of Zika?
VESPIGNANI: Yeah, I agree with the WHO statement on this.
DOUCLEFF: The Centers for Disease Control and Prevention's director, Dr. Thomas Frieden, told reporters that travelers to the Olympics will make up just a tiny fraction of people coming in and out of Zika-infected areas.
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THOMAS FRIEDEN: So even if you were to say the Olympics weren't to happen, you'd still be left with 99.75 percent of the risk of Zika continuing to spread.
DOUCLEFF: So right now, the CDC agrees the games should stay in Rio. Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.