The UN's health agency warned Monday of the potential for a "marked increase" in Zika infections, and the spread of the virus to new parts of the world, even as the outbreak declines in Brazil.
Largely contained to Latin America and the Caribbean, Zika's range is likely to expand as summer arrives in the northern hemisphere, and with it virus-transmitting mosquitoes.
"As seasonal temperatures begin to rise in Europe, two species of Aedes mosquito which we know transmit the virus will begin to circulate." World Health Organization assistant director general Marie-Paule Kieny told a Zika science conference in Paris.
Add to that the risk of Zika-infected men passing the virus on to women via sex, and the world "could see a marked increase in the number of people with Zika and related complications," Kieny said.
At the same time, with cooler temperatures in the tropics and subtropics, the outbreak in hardest-hit Brazil was "clearly on the decline." she added, without providing numbers.
Recent scientific consensus is that Zika causes microcephaly, a form of severe brain damage in newborns, and adult-onset neurological problems such as Guillain-Barre Syndrome, which can cause paralysis and death.
"It's not what we know but what we don't know that is concerning," said infectious diseases professor David Heymann of the London School of Hygiene and Tropical Medicine.
"We can't make recommendations (for prevention) if we don't understand the full potential of a virus or bacteria."
There is no vaccine or treatment for the virus, which in most people causes only mild symptoms like a rash, joint pain or fever.
Developers in the United States, France, Brazil, India and Austria are working on 23 vaccine-development projects, Kieny said.
A Zika outbreak began in Brazil in early 2015, followed nine months later by a surge of infants born with microcephaly, and an increase in Guillain-Barre cases.
Brazil reported some 1.5 million infections out of an estimated global total of two million in more than 40 countries.