GENEVA — The World Health Organization said Friday that the outbreak of Zika remains an international health emergency and that while the virus continues to hit new countries, experts still aren't sure how big the risk is that pregnant women who catch the virus will give birth to brain-damaged babies.
The UN health agency convened its expert committee this week to assess the latest status of the epidemic.
WHO officials said Friday that no Zika infections were reported in Brazil during the Olympics, either among athletes or visitors.
Brazil presented data that convinced WHO experts that no infections had occurred either during the Games or after visitors and athletes had returned to their home countries.
"They gave us very convincing data," said Dr. David Heymann, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine, who led the panel of experts. He said health facilities in Brazil had been poised to track and treat Zika cases, but had reported none.
In the absence of any effective treatments or vaccines for the disease — and given past failures to wipe out the mosquitoes that mostly spread Zika — Heymann said it will largely be up to individuals to avoid infection.
''We don't have a definitive answer,'' said Dr. Peter Salama, WHO's director of emergencies. ''The risk is relatively low, but significant.''
Despite Zika's spread to more than 70 countries and territories, Brazil has the vast majority of cases of microcephaly, or infants born with abnormally small heads. Heymann said that studies are ongoing in the country and that the explanation could involve numerous factors.
''It could be all the way from genetic [factors] to nutritional to environmental contaminants,'' he said.
Salama said officials are still trying to figure out whether the two known strains of the disease both cause microcephaly. So far, it is primarily the Asian strain of Zika, which is circulating in the Americas, that has been definitively linked to the severe birth defects.
In recent months, officials in Guinea-Bissau reported several microcephaly cases shortly before Zika was officially detected. Salama said that while Zika samples from the country appear to be from the African strain, it hasn't been determined whether the African strain of the virus might also be responsible for the neurological problems.
Scientists have known about Zika since the 1940s, but its explosive spread in Latin America began only recently. Symptoms in adults are mild, but the virus can cause severe birth defects if pregnant women are infected, so it is being tracked closely.
It moved from Brazil to Colombia and is now spreading fast in the Caribbean, particularly in Puerto Rico, where there are more than 14,000 infections, including in nearly 1,000 pregnant women. In the continental United States, small clusters have occurred in South Florida.