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Rio Olympics 2016: the CDC says Zika risk is low

The Centers for Disease Control and Prevention just released a new Olympics Zika risk assessment, and it’s filled with good news: The Summer Games, to be held in Brazil in August and September, are unlikely to spur additional spread of the virus. And of the 206 countries participating, only four face an increased risk of importing new Zika cases.

The key reasons officials aren’t worried about mass outbreaks stemming from the games in Rio: It’s the cooler winter season there, when mosquitoes aren’t out biting people — which is the main way the Zika virus spreads. And Olympic travelers represent only a small fraction (less then 0.25 percent) of people traveling into and out of Zika-infected countries.

So health experts have been saying the Olympics shouldn’t pose a unique risk of Zika contagion.

To better understand the risk, the CDC’s new model considered a few factors related to the conditions in places where participating athletes were coming from: whether their home countries have seen Zika outbreaks, whether they have the mosquito (Aedes aegypti) that carries the virus and a season of active transmission, and how much travel a country has already had with affected regions.

They then put together a "worst-case scenario," based on several assumptions: that Zika would continue transmitting during Brazil’s winter; that preventive measures — like wearing mosquito repellent and long sleeves — would fail; that infected visitors would still be contagious when they returned home; and that those infected people would not take precautions to prevent mosquito bites back home, either.

Assuming all that, they concluded that of the 206 countries participating in the games, four — Chad, Djibouti, Eritrea, and Yemen — are at risk of potential Olympic-related Zika importations.

The main reasons: Not only will travelers from these countries go home to mosquito season, where the Aedes aegypti can spread the virus, but Olympic travel to Brazil will represent a unique exchange with a Zika-infected country — something that hasn’t been happening otherwise.

Still, the overall likelihood of any outbreak is small. The researchers noted that there are only 19 athletes and a delegation of 60 people coming from these four countries, and all of the above assumptions would need to hold true to get even one imported case.

"With the exception of these four countries, the Games do not pose a unique or substantive risk for mosquito-borne transmission of Zika virus in excess of that posed by non-Games travel," the researchers added.

Read more from Vox about Zika here.