The Zika Olympics – The Atlantic

A paper published in April calculated the probable number of Zika cases during the Olympics, using dengue transmission during the 2008 outbreak as a model. It found that, on the low end, there would be 1.8 cases per one million tourists, and on the high end, 3.2 cases per 100,000 tourists.

So the concern is not necessarily that tourists will fall ill while they’re at the games. (Though everyone seems to agree that pregnant women, at least, should stay away.) But “while Zika’s risk to any single individual is low, the risk to a population is undeniably high,” the letter reads. The fear is that travelers will bring the virus home, either in their bodies or in the bodies of mosquito stowaways, and it will spread further.

“People talk about that all the time,” says Jane Messina, an epidemiologist at the University of Oxford, of the chance of infected mosquitoes hopping on airplanes. “Yeah, they could get on an airplane, I suppose, but I assume any airplane leaving Rio will spray. At the end of the day people move viruses internationally. Mosquitoes don’t, really.”

Large global gatherings do carry some risk of spreading disease—for example, in 2000 and 2001, the Hajj, the annual Muslim pilgrimage to Mecca, was associated with an outbreak of meningococcus. The CDC has a whole webpage on advice for staying healthy during “mass gatherings.” But it’s still hard to predict the impact of any one gathering affected by any one disease.

And Zika may have already had another, similar opportunity two years ago. Contrary to a popular theory that the virus arrived in Brazil during the 2014 World Cup, a paper published earlier this year in Science estimated a 2013 arrival date for Zika by tracking genome mutations and using epidemiological data. In that case, Zika was already around for one big global event in Brazil. Whether the World Cup played a role in spreading Zika is unclear, but “there’s been a lot of opportunity for Zika to get brought over from Brazil already,” says Messina, who has worked on global risk maps of Zika.