In January 2016 the World Health Organization (WHO) declared Zika virus a Public Health Emergency of International Concern because of its rapid spread and its association with a severe deformity known as microcephaly (abnormal smallness of the head) in infants born to mothers infected early in pregnancy. The virus had given rise to a significant outbreak of Zika fever in Brazil in 2015, which saw a dramatic rise in neurological syndromes, including microcephaly and Guillain-Barré syndrome. By early 2016 the virus had spread to Puerto Rico and Mexico, and by the end of July the first cases of local transmission had been reported in the continental United States, just north of downtown Miami. Up to that time, all U.S. cases of Zika fever had been in persons who had recently traveled to regions with active Zika transmission—places populated by Aedes aegypti mosquitoes, the agents responsible for carrying and transmitting the virus to humans.
As many as 500,000 visitors were expected to converge on Rio for the 2016 Olympic Games, raising concerns about the possibility of furthering the international spread of Zika. Travelers who became infected in Brazil could potentially take the virus to previously unaffected countries. Of special concern were countries already inhabited by A. aegypti, such as those in Africa, Asia, and the Americas, since the virus could spread quickly by mosquitoes once introduced.
In the week before the start of the 2016 Olympics, however, new research showed that the risk of Zika infection and transmission among travelers was low. Analyses predicted that between 3 and 37 travelers would become infected. Moreover, the majority of travelers would be returning to countries where the risk of mosquito transmission was negligible. Rather, infected men having unprotected sex would be the most likely means of Zika transmission in those places. Many travelers were also expected to take individual precautionary measures and would be staying in hotels where screens and air-conditioning would make mosquito transmission unlikely.