GENEVA -- The World Health Organization says 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 U.N. health agency convened its expert committee this week to assess the latest status of the epidemic.
Dr. David Heymann, the committee's chairman and an infectious-disease professor at the London School of Hygiene and Tropical Medicine, said Friday that considerable gaps remain in understanding Zika and the complications it causes -- including babies with serious neurological problems -- and WHO concluded that the outbreak remains a global emergency.
"This extraordinary event is rapidly becoming, unfortunately, an ordinary event," Heymann said, explaining that health officials around the world should prepare for the imminent arrival of the disease spread mostly by mosquitoes but also through sex.
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.
"People have to assume responsibility for this on their own," he said, adding that people at risk of the disease should wear long sleeves and insect repellent.
WHO said it was also unknown just how big the risk is for pregnant women.
Although Zika has been proved to cause a range of neurological problems in babies, various studies have put the risk anywhere from 1 to 30 percent.
"We don't have a definitive answer," said Dr. Peter Salama, WHO's director of emergencies.
"The risk is relatively low, but significant."
One of the most fundamental questions in this current epidemic is why countries such as Brazil, the epicenter of the current epidemic, have reported nearly 2,000 cases of microcephaly and other brain abnormalities, while countries such as Colombia have reported fewer than three dozen cases.
Scientists need to examine possible factors that could affect Zika's damage, including genetic factors, environmental contaminants, and other co-infections, Heymann said.
"A whole range of cofactors must be eliminated to say, with certainty," that the only culprit is the Zika virus, he said.
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 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 researchers need to understand whether the time lag between when an outbreak occurs to when birth defects show up nine months later could account for country-by-country differences, or "is it truly something to do with cofactors."
Salama said officials are also 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.
WHO officials also said Brazil reported no confirmed cases of Zika after the Rio de Janeiro Olympics.
That data is based on information collected by authorities on people who sought treatment at health facilities who were athletes or spectators.
Most people with Zika don't have symptoms, but Salama said enhanced surveillance by WHO of the Olympic delegations show no large-scale transmission of Zika, confirming the earlier assessments of WHO and the Centers for Disease Control and Prevention that the Olympics posed no significant additional risk of spreading Zika.
A study published Thursday identified eight countries in Asia and Africa that researchers say are at the greatest risk of Zika virus transmission.
The countries -- India, China, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan and Bangladesh -- all have hot and humid climate conditions; the type of mosquitoes that primarily spread the virus; a high volume of travelers returning from Zika-affected areas in the Americas; and large, dense populations. And several have limited health care resources.
Information for this article was contributed by Jamey Keaten and Maria Cheng of The Associated Press; and by Lena H. Sun of The Washington Post.
A Section on 09/03/2016