The 2016 Olympics are highly unlikely to spread Zika worldwide, federal officials said, noting that the 500,000 people expected to travel to Brazil for the August games account for less than 1% of all international travel to Zika-affected areas.

Visitors to Brazil will have a relatively low risk of contracting the mosquito-borne illness because the Olympics will take place during winter in the Southern Hemisphere, according to a new analysis from the Centers for Disease Control and Prevention. The Paralympic Games will be held in September, when the weather in Brazil also remains cooler and dryer.

Only 19 of the more than 200 countries whose citizens are expected to visit Brazil for the Olympics are vulnerable to Zika outbreaks related to travel to the games, according to the CDC. That’s because those countries don’t yet have Zika outbreaks, but have climates and mosquito populations that could allow the virus to spread. But because most of those countries already have so much travel to Zika-affected regions, the slight increase in travel to Brazil for the Olympics won’t dramatically affect their risk.

Only four countries — Chad, Djibouti, Eritrea and Yemen — have a major risk of Olympics-related Zika outbreaks, according to the CDC. That’s because the Olympics will represent the only major travel from those countries to a Zika-infected area. The four nations are expected to send a total of 19 athletes to Rio, who will be accompanied by delegations of 60 additional people.

More than 200 health experts have called for the Olympic games to be canceled because of the risk of Zika spreading worldwide. Brazilian health officials estimate that up to 1.5 million people in that country have been infected with the virus, which can cause devastating birth defects in babies infected while in the womb.

The World Health Organization concluded the Olympics doesn’t pose a major risk of spreading Zika.

The CDC continues to warn pregnant women to avoid traveling to Zika-affected areas, including Brazil, because of the risk that the virus will harm their fetuses. Zika can cause devastating brain damage to fetuses, including a condition called microcephaly, in which babies are born with abnormally small skulls and, in most cases, incomplete brain development.

All visitors to the Olympics Games should take steps to prevent mosquito bites, both during their trip and for three weeks after they return home, according to the CDC. Although returning travelers may not feel sick, it’s possible they could still be infected with Zika and therefore able to spread the virus to mosquitoes in their home countries. Those mosquitoes could then incubate the virus and begin spreading it to others.

The CDC also cautions people who visit Brazil to take steps to avoid contracting the disease sexually. Men infected with Zika can spread the virus to their sexual partners. It’s not known if infected women can spread the disease through sex.

More than 1,100 travel-related Zika cases have been diagnosed in the continental U.S., including 320 cases in pregnant women, according to the CDC.

In Puerto Rico and other U.S. territories, where Zika is spreading among local mosquitoes, doctors have diagnosed 2,534 cases of Zika, including 279 in pregnant women.

The actual number of Zika cases could be much higher. That’s because only about 20% of people infected with the virus develop symptoms, which include a rash, fever, joint pain and pink eye.