Pregnant women are advised not to travel to Brazil during the 2016 Olympic Games to avoid potential birth defects related to an outbreak of the mosquito-borne Zika virus, President Dilma Rousseff’s chief of staff, Jaques Wagner, said.

“It’s a serious risk for pregnant women,” Wagner told reporters after an emergency cabinet meeting Monday. He added that “it’s not recommended” for them to travel to Brazil.

The risk for other adults is relatively small, and most people never even show symptoms, Wagner said. He said the chance of the Olympics being canceled “doesn’t exist.”

The Zika virus in Brazil has been linked to more than 4,000 suspected cases of microcephaly, a condition that causes babies to be born with abnormally small heads and development problems. Rousseff welcomed the World Health Organization’s decision to declare a public health emergency about the virus because it will help raise awareness, Wagner said.

The WHO has estimated that there could be 3 million to 4 million cases of the Zika virus in Latin America. The travel industry has begun to feel the impact of the virus’ outbreak as worried vacationers and business customers avoid the affected areas. Cases have been found in at least 23 countries and territories in the Americas as of Jan. 28.

There is no vaccine for the Zika, which has long been endemic in Southeast Asia and parts of Africa where many people have developed immunity. It is new in the Americas, however, and a treatment or a vaccine it could take years to be available. As an alternative, health officials are emphasizing mosquito control to try and stop the virus from spreading.

“This has to be a long-term effort, or the mosquito will return,” Wagner said. “For now, the only vaccine is awareness.”

–With assistance from Drew Armstrong.