Zika virus belongs to the genus Flavivirus of the family Flaviviridae; it is transmitted to humans primarily through the bite of an infected Aedes species mosquito (e.g., Ae. aegypti and Ae. albopictus) (1). Zika virus has been identified as a cause of congenital microcephaly and other serious brain defects (2). As of June 30, 2016, CDC had issued travel notices for 49 countries and U.S. territories across much of the Western hemisphere (3), including Brazil, where the 2016 Olympic and Paralympic Games (Games of the XXXI Olympiad, also known as Rio 2016; Games) will be hosted in Rio de Janeiro in August and September 2016. During the Games, mosquito-borne Zika virus transmission is expected to be low because August and September are winter months in Brazil, when cooler and drier weather typically reduces mosquito populations (4). CDC conducted a risk assessment to predict those countries susceptible to ongoing Zika virus transmission resulting from introduction by a single traveler to the Games. Whereas all countries are at risk for travel-associated importation of Zika virus, CDC estimated that 19 countries currently not reporting Zika outbreaks have the environmental conditions and population susceptibility to sustain mosquito-borne transmission of Zika virus if a case were imported from infection at the Games. For 15 of these 19 countries, travel to Rio de Janeiro during the Games is not estimated to increase substantially the level of risk above that incurred by the usual aviation travel baseline for these countries. The remaining four countries, Chad, Djibouti, Eritrea, and Yemen, are unique in that they do not have a substantial number of travelers to any country with local Zika virus transmission, except for anticipated travel to the Games. These four countries will be represented by a projected, combined total of 19 athletes (plus a projected delegation of about 60 persons), a tiny fraction of the 350,000–500,000 visitors expected at the Games.* Overall travel volume to the Games represents a very small fraction (<0.25%) of the total estimated 2015 travel volume to Zika-affected countries,† highlighting the unlikely scenario that Zika importation would be solely attributable to travel to the Games. To prevent Zika virus infection and its complications among athletes and visitors to the Games and importation of Zika virus into countries that could sustain local transmission, pregnant women should not travel to the Games, mosquito bites should be avoided while traveling and for 3 weeks after returning home, and measures should be taken to prevent sexual transmission ( Box).

To assess the country-specific risk for importation and sustained, local mosquito-borne transmission of Zika virus from travel to the Games, CDC constructed a stepwise model. The model began with the 206 countries and numbers of athletes planning to participate in the Games, as of June 30, 2016 (5). Each country was assessed on five criteria: 1) no active CDC travel notice (as of June 30, 2016) reporting local Zika virus transmission (3); 2) modeled probability of Ae. aegypti presence, drawn from a data set of 20,000 observed occurrences during 1960–2014 (6) and fitted to climate norms for the months of August and September when travelers would return to their home country; 3) predicted dengue epidemic potential (7), such that the environmental and population conditions could support mosquito-borne disease spread from a single point of introduction; 4) lack of historic Zika virus circulation as evidenced by historic serosurveys, Zika virus detection, or exported Zika virus disease cases to exclude any country in which Zika virus might already be endemic (8); and 5) ranking countries by the estimated aviation travel passenger-journeys during August 2016 from Rio de Janeiro. Combined, the first four criteria estimate susceptibility to ongoing Zika transmission from introduction by a single traveler to the Games or to any other country with ongoing Zika virus transmission during August–September. The fifth criterion considers the probability that ongoing transmission could be the result of travel to the Games alone. In a stepwise manner, CDC successively excluded countries that did not meet the preceding criteria ( Table 1).

Research based on the previous four summer Olympics has indicated that travel during the Olympics typically does not exceed baseline travel volume patterns to the host city (Kamran Khan, Li Ka Shing Knowledge Institute, St. Michael’s Hospital, Toronto, Canada, personal communication, June 2016). Therefore, for most countries, the number of passenger-journeys from Rio de Janeiro during August 2016 was assumed to be approximately the same as the number of journeys occurring during August 2015. Modeled data were used to estimate the number of passenger-journeys originating from Rio de Janeiro during August 2015.§ For the countries without documented passenger-journeys from Rio de Janeiro in August 2015, the estimated Olympic delegation size¶ was used as a proxy for travel volume during August 2016.** Delegation size was determined by viewing video footage of the Parade of Nations from the London 2012 Olympics Opening Ceremony and comparing the number of persons observed with the known number of athletes; this is likely an underestimation given that some delegates might not participate in the parade (5). Finally, CDC calculated the estimated aviation travel from Rio de Janeiro during August 2016 as the proportion of each country’s total travel to all Zika-affected countries during 2015.†† For example, there were 36,923,504 passenger-journeys to the United States from all Zika-affected countries and U.S. territories in 2015 and 38,798 journeys from Rio de Janeiro during August 2015; thus, the proportion of estimated U.S. travel from Rio de Janeiro for the Games, relative to that of all Zika-affected countries is 0.11%. CDC assumed that any country whose estimated proportion of travel to Rio de Janeiro among all travel to Zika transmission areas exceeded 5% was at risk for ongoing transmission of Zika transmission attributable to the Games.

Among the 206 countries competing in the Games, 39 have CDC travel notices indicating ongoing Zika virus outbreaks or epidemics (Table 1). Among the remaining 167 countries, 148 were not considered to be at risk for the following reasons: 74 did not have a predicted presence of Ae. aegypti in August–September, 51 did not also have a predicted high dengue epidemic potential, and 23 also had evidence of previous Zika virus transmission. Thus, 19 countries currently not reporting Zika outbreaks that are participating in the Games met all the risk criteria for susceptibility to ongoing Zika transmission from introduction by a single traveler to the Olympics. For 15 of these countries, estimated aviation travel from Rio de Janeiro in August 2016 compared with total aviation travel from all countries with local Zika virus transmission in 2015 was 0.01%–3.25% ( Table 2). Four countries (Chad, Djibouti, Eritrea, and Yemen) were estimated to have >19% of travel from Rio de Janeiro in August 2016 compared with the overall aviation travel from all countries with local Zika transmission.