The viral disease carried by mosquitoes has spread to more than 60 countries and territories since an outbreak was identified in Brazil in 2015, raising alarm over its ability to cause microcephaly as well as Guillain-Barre syndrome.
"The prevalence of Zika is dropping, certainly in the Americas," Ian Clarke, WHO incident manager for Zika, told a news briefing.
He said it was not clear why infection rates were falling.
"The anticipation was that we would see a second wave, certainly in Brazil and we haven't seen it. And we don't believe this is because we're not looking. There's a lot of surveillance ongoing," Clarke said.
Further research was needed into issues including whether there was a "natural immunity" due to previous exposure.
Wednesday marked the first anniversary of WHO declaring that Zika constituted an international emergency. It lifted that designation in November while maintaining its advice for pregnant women and travelers.
To date, 29 countries have reported thousands of babies born with Zika-linked microcephaly, which causes abnormally small-sized heads, often signifying arrested brain development.
Long-term efforts were required, such as against other infectious diseases including malaria and dengue fever.
"We should expect to see outbreaks. We saw it in Singapore, we will see it again," Clarke said.
Angola said last month it had recorded its first two cases of Zika virus.
"In a couple of other countries we're doing some quite intensive surveillance work, Tanzania and some others where we know there is the vector (the aedes aegypti mosquito), we know that there is chikungunya, so really looking to see if we can find Zika," he said.
There is no preventive treatment against Zika, but drug companies are rushing to develop a vaccine.
"There are about 40 vaccine candidates, five are entering or about to enter in phase 1, clinical trial for safety and ability to induce an immune response," said WHO's Dr Bernadette Murgue.
The five experimental vaccines, as listed on the WHO website, are made by: Gene One Life Science Inc/ Inovio Pharmaceuticals Inc; Moderna Therapeutics; and by the U.S. National Institute of Allergy and Infectious Diseases.
"It will take at least two to three years before registration of these vaccines," Murgue said.
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