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Brazil’s hosting of the Olympics next month has prompted concern that the games could propel the spread of Zika across the globe. But the event in Rio de Janeiro is unlikely to cause the virus to spread to new places, according to a new report from the Centers for Disease Control and Prevention.

The CDC’s risk assessment, published Wednesday, bolsters the case that the games should not fuel much wider spread of the mosquito-borne Zika virus, which can cause birth defects when it infects pregnant women. A key reason: Even though hundreds of thousands of people are expected to head to Rio, they represent only a small percentage of the overall travel to and from areas with Zika transmission.

It is also winter in Rio, so there will be less mosquito activity and thus decreased disease transmission during the games, experts say.


For the report, global health experts developed a model based on travel patterns to Zika-afflicted areas, whether countries had the right conditions and mosquitoes for local spread, and whether countries were sending athletes to the games.

The analysis identified 19 countries that as of now do not have local Zika transmission but that could sustain the virus if a traveler returned home infected from the games. But 15 of them already have substantial travel to Zika-affected countries, meaning they have a higher likelihood of importing Zika through people traveling to the region for reasons outside the Olympics.


The remaining four countries — Eritrea, Yemen, Djibouti, and Chad — have no substantial travel to Zika-affected regions outside the games.

“With the exception of four countries, attendance at the games does not pose a unique or substantive risk for mosquito-borne transmission of Zika virus in excess of that posed by non-games travel,” the authors wrote.

The study did not try to address the question of whether, or to what extent, the Olympics could exacerbate Zika outbreaks in any country.

Collectively, the four at-risk countries identified in the report are expected to send only 19 athletes and a delegation of 60 people to Rio for the games, meaning the absolute risk of an athlete or support staff contracting Zika and then seeding local transmission once home is relatively small.

Brazilian officials have said they expect as many as 500,000 international visitors for the Olympics and Paralympics. That sounds like a lot of people who could contract Zika from mosquitoes in Brazil — the country hardest hit by the Zika virus — and then carry the virus back home.

But consider this: There were almost 37 million journeys to the United States alone from Zika-affected countries and US territories in 2015, according to the CDC report, showing just how small of an impact Olympic-related travel has on larger global travel patterns.

“Globalization in the 21st century is the primary driver of Zika virus spread that has occurred to date,” said Dr. Martin Cetron, an author of the report and the director of the CDC’s Division of Global Migration and Quarantine. “It’s really driven by global aviation and travel and connectivity. And the relative contribution of the Olympics in Rio is quite small compared to that overall risk.”

Since May 2015, travel has helped transmit the Zika virus throughout much of the Americas and the Caribbean and is responsible for almost all of the more than 1,100 Zika cases so far in US states. (The only other cases in the United States were acquired through sex; there has been no local mosquito-driven spread seen as of now.)

Some people have been calling for the games to at least be delayed, and some athletes have announced they won’t travel to Rio.

But global health officials have been saying for months that the games should not accelerate the international spread of the virus; the World Health Organization said explicitly in May that “there is no public health justification for postponing or canceling the games.”

Cetron said that bioethicists who think the Olympics and Paralympics should be canceled because of Zika are mistaken, “as if you could turn off the switch on the games and you could halt Zika virus in its tracks.” In fact, eliminating Olympic-related travel would address less than 1 percent of the annual travel from areas affected by Zika to the rest of the world, according to the report.

People who do go to the games are encouraged to take steps to avoid mosquito bites while in Brazil, and for three weeks when they return home to avoid infecting mosquitoes there and starting a chain of local transmission. The WHO and CDC have said pregnant women should not travel to the Olympics or to any area where the virus is spreading.

The authors of the CDC report acknowledge some limitations with their analysis, including their reliance on estimates of travel data, not the raw number of journeys among different countries.