A new risk assessment conducted by the Centers for Disease Control and Prevention (CDC) has found that the Olympic and Paralympic games do not pose a ‘unique or substantive risk’ for mosquito-borne Zika transmission in excess of that posed by non-Games travel.

After performing a stepwise model, the CDC found that Zika importation – to any country not on the active CDC travel notice – solely attributable to the Games in Rio, is an ‘unlikely scenario’. Overall, travel to the Games is substantially dwarfed by overall travel to Zika-affected countries in 2015 (<0.25%).

The risk level will be greater for mosquito-borne transmission of the Zika virus for four countries; Chad, Djibouti, Eritrea, and Yemen. The CDC estimated that a total of 19 countries with no record of a Zika outbreak, have the environmental conditions and susceptibility to sustain a mosquito-borne transmission of the virus, however the risk level will only increase beyond what usual aviation travel incurs for the four aforementioned countries. 

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“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,” states the report.

Including athletes and delegates, these four countries are expected to bring 79 persons to the Games, a very small fraction of the estimated 350–500,000 expected to visit.

To identify the countries with no record of Zika but with the environmental conditions to sustain an outbreak, the CDC’s stepwise model analyzed each of the 206 countries with competing athletes on five criteria:

1. No active CDC travel notice (as of June 30, 2016) reporting local virus transmission.

2. Modeled probability of Ae. aegypti presence (the mosquito species which carries Zika).

3. Predicted dengue epidemic potential.

4. Lack of historic Zika virus circulation.

5. Ranking contries by the estimated aviation travel passenger-journeys during August 2016 from Rio de Janeiro.

The CDC’s model was based on five ‘worst case scenerios’, 1) assuming that Zika transmission would be ongoing during the usually low mosquito season of August and September in Rio; 2) assuming that preventative measures such as long sleeves and repellent would not be used; 3) assuming that all infected visitors would be viremic upon returning to their home country; 4) infected visitors were assumed to immediately return to their home country; and 5) it was assumed that those infected visitors would not use preventative caution against mosquito bites once they return home.

Although the report asserts that for most countries the Games do not pose any greater risk level of Zika transmission than that posed by usual travel, it reiterates that, “Public health interventions that increase ongoing readiness and response capabilities to prevent Zika virus transmission,” should be implemented.

The report concludes, “All delegation members and visitors should take rigorous steps to reduce the likelihood of mosquito bites (e.g. use insect repellent) both during the Games and within the three weeks after they return to their home country from an area with ongoing Zika transmission.”

For more information visit CDC.gov.