Even though a number of high-profile athletes are citing the Zika virus as their reason for pulling out of the Olympic Games in Rio de Janeiro next month, experts say the risk of international spread due to the event is low.
In a study published Monday in the Annals of Internal Medicine, researchers predict that in the worst case, six to 80 travelers will contract the virus in Rio, out of 350,000 to 500,000 expected to attend the games. Accounting for the 10-day period when Zika is detectable in the blood, the number of people expected to bring the virus back to their home countries is three to 37. (The games last 17 days, so people who acquire the infection could be rid of it before they return home.)
The researchers base their model on the assumption that visitors will be exposed to the same levels of infection as local residents, though they believe exposure will actually be lower, with tourists more likely to stay in air-conditioned, screened-in hotels.
A co-author of the study, Gregg Gonsalves — who co-directs the Global Health Justice Partnership between Yale Law School and the Yale School of Public Health — says a number of other factors are likely to lower travelers’ risk.
“The epidemic is much more extensive in the northern areas of the country than it is down in Rio,” Gonsalves said. He added that it’s currently winter in Brazil, which is not peak mosquito season, and that most of the games will take place indoors.
Based on the distribution of travelers who attended the World Cup in Brazil in 2014, the researchers expect a majority of foreign visitors to come from the U.S., Canada, Japan, Europe, and other countries where the risk of mosquito-borne transmission is low.
Last week, the first two possible cases of non-travel-related Zika virus transmission were reported in two Florida counties, Broward and Miami-Dade, though according to NBC, sexual transmission — as opposed to transmission by a mosquito here in the U.S. — has not been ruled out in either case. Still, Sahotra Sarkar, a disease ecologist at the University of Texas at Austin, says local transmission in the state was to be expected.
“We predicted that those areas of Florida are the ones with the greatest risk,” he said. “The next area besides Florida where we expect we will see local transmission would be Texas, probably in the Houston area.”
Sarkar emphasized that the extent to which sexual transmission plays a role in spreading the disease is still not well understood. But visitors to the Olympics are being urged to practice safe sex, as the virus has been detected in semen up to 62 days after symptoms became apparent. Pregnant women — who are most at risk — are cautioned not to attend at all.
“We want to continue to make sure that we push the messages that we’ve been pushing all along and that’s making sure pregnant women don’t travel to areas where Zika is circulating,” said Tom Skinner, a senior public affairs officer at the CDC.
Skinner says travelers should take precautions to avoid mosquito bites and monitor themselves for any symptoms once they return home. For most people, Zika causes only a minor illness, with fever and possible joint pain, headache and rash lasting up to a week after exposure. But for pregnant women, the virus can lead babies to be born with microcephaly, a devastating birth defect that causes the head to be abnormally small.
So far, Zika concern has been focused mainly on the Aedes aegypti mosquito, known to spread the dengue and chikungunya viruses. But following speculation this year that the more widespread Culex genus might also be a vector, researchers from the Brazilian institute Fiocruz announced on Thursday that they had detected Zika in wild-caught Culex mosquitoes.
Not yet published, the results show that Zika was detected in three of 80 groups of Culex mosquitoes collected around the Brazilian city of Recife, more than 1,100 miles north of Rio de Janeiro.
The finding highlights the need for more research, but will not change the current Zika strategy.
The Culex report “was not completely convincing,” Sarkar said. “What the report really showed is that the virus can be found in the mosquito; whether it’s found in such great concentrations to transmit the disease remains unclear. We are modeling it and other people are looking at it, but that’s not yet a reason for concern.”
Constância Ayres, who led the Fiocruz study, said in an email that she agreed with that assessment but added that cities with Culex mosquitoes should be aware of the potential risk.
While the Olympics draws a lot of attention, Gonsalves’s article points out that the number of visitors it’s expected to bring to Brazil account for only a fraction of the more than 6 million people who travel to the country each year.
And as Kaiser Health News reported last week, the majority of travel-related cases in the U.S. have come not from Brazil but from the Dominican Republic, reflecting visits to see family members, rather than tourism.
“I think the case we’re making is that the Olympics are the least to be worried about,” Gonsalves said.