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
[...] With the exception of [Chad, Djibouti, Eritrea, and Yemen], 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 CDC mentions that the assessment is based on worse case scenario projections that include assuming maximal seasonal transmission, no anti-mosquito precautions, and returning to the home country at the point of maximal viremia.
http://www.cdc.gov/mmwr/volumes/65/wr/mm6528e1.htm [cdc.gov]