Speaking after a meeting of the WHO's emergency committee on Zika, its chairman David Heymann said the disease continued to constitute an international health emergency although the risk to people attending the Paralympics in Rio de Janeiro this month was low.
"This extraordinary event is rapidly becoming, unfortunately, an ordinary event. And Zika is beginning to spread, continuing to cause outbreaks in many countries around the world," Heymann told a news conference.
Singapore, with a "very technologically advanced health system", was able to identify the disease "very early", Heymann said. "But in other countries where it might enter at some time, that might not be the case."
Peter Salama, head of disease emergencies and outbreaks at the WHO, said the virus was likely to keep spreading.
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Florida officials on Thursday said they have trapped the first mosquitoes that tested positive for the Zika virus in the Miami area, further confirming reports of local U.S. transmission of the illness. There have been 49 cases of Zika in people believed to have contracted the virus in a small area of Miami
Salama, asked about Florida and the risk of spread elsewhere in the United States, said: "In terms of further spread, yes, a risk. As we said, the U.S. is no exception. Wherever there is a competent vector, in this case the Aedes aegypti (mosquito), there is a risk that the virus will spread."
Zika infections in pregnant women have been shown to cause microcephaly - a severe birth defect in which the head and brain are undersized - as well as other brain abnormalities. Brazil has confirmed more than 1,800 cases of microcephaly in the past year.
Although there is no vaccine and scientists don't yet know how to stop Zika, they foresaw a low risk of the virus spreading at the Olympic Games last month in Brazil, the country hardest hit by the virus, during the winter season there.
Nobody returning from the Games, neither travellers nor athletes, has reported Zika symptoms, although most cases of the disease are asymptomatic, Salama said.
"Certainly we feel fairly confident that the risk assessment that there would be no significant increased transmission due to the Olympics is very much on track," he said.
Singapore, now with 189 known cases, has had a textbook response with transparency and quick reporting, Salama said.
"The way the government of Singapore has handled the outbreak in Singapore really represents in many cases a role model...remembering that the first case was only confirmed really just a little over a week ago," he said.
Heymann said Singapore was expected to complete genetic sequencing of the virus by next week, which would show which strain was causing the outbreak there.
(Reporting by Stephanie Nebehay, editing by Tom Miles and Mark Trevelyan)
