"If that rate continues, we expect more than 2,500 cases will emerge of babies with brain damage and clinical signs of microcephaly," said Anthony Costello, who heads WHO's department of maternal, newborn, child and adolescent health.
WHO Director-General Margaret Chan said new evidence is becoming clear daily about the mosquito-borne virus. A pattern seems to be emerging: Initial detection of Zika is followed within about three weeks by an unusual increase of Guillain-Barré syndrome, a rare condition that can cause paralysis and sometimes death. Detection of microcephaly and other fetal abnormalities typically comes about six months later, WHO officials said, as pregnancies of infected women come to term.
Brazil and Panama are now reporting cases of microcephaly -- including 6,480 suspected cases in Brazil. Panama has one reported case. Colombia is investigating many cases for a possible link. A WHO team is currently in Cape Verde to investigate that country's first reported case of microcephaly, Chan said.
A total of 12 countries and territories have now seen an increase in the incidence of Guillain-Barré or actual lab confirmation of Zika among cases with the syndrome, she said.
Although Zika has hit countries in Latin America and the Caribbean the hardest, Chan warned that no one can predict if it will spread to other parts of the world and trigger the same pattern of health complications. In other affected countries, the virus has not been circulating long enough for pregnancies to come to term, she said.
"If this pattern is confirmed beyond Latin America and the Caribbean, the world will face a severe public health crisis," she said. Chan said the Zika virus initially looked "reassuringly mild," with no hospitalizations or deaths reported when it first showed up in Brazil last May.
But in less than a year, she said, "the status of Zika has changed from a mild medical curiosity to a disease with severe public health implications." The possibility that a mosquito bite could be linked to severe fetal abnormalities "alarmed the public and astonished scientists," she said.
The world community needs to shift thinking and preparations for the long term, she said. Countries need to build health capacity and social support to help families cope with the "heartbreaking strain" from caring for such babies.
The Cape Verde archipelago off West Africa has more than than 7,000 Zika infections, including its first case last week of a newborn with microcephaly.
Although the link between Zika and fetal abnormalities has not been proven, "the circumstantial evidence is now overwhelming," she said. Urgent action should not wait for definitive proof, she added.
Recent studies have detailed how the pathogen could attack fetal brain development as well as trigger complications at any stage of pregnancy.
In a study published last week the Lancet medical journal, researchers analyzed a 2013-2014 Zika outbreak in French Polynesia and said the risk of microcephaly is about one for every 100 women infected with the virus during the first trimester of pregnancy. The research, based on statistical and mathematical models, was conducted after the outbreak was over.
Researchers cautioned, however, that outbreaks differ, and the risk will depend on how the virus spreads and the makeup of the affected population. WHO officials said they don't have enough information yet to say what the risk of microcephaly is for pregnant women in Brazil or elsewhere. Between 2001 and 2014, Brazil had, on average, about 163 cases of microcephaly a year.
The global health agency has convened three meetings in recent weeks to receive updates on the science, mosquito control tools and management of health complications. Chan said the most urgently needed medical priority is a reliable diagnostic test for Zika. More than 30 companies are working on, or have developed potential new tests for the virus. The U.S. Food and Drug Administration last week approved a tool developed by the Centers for Disease Control and Prevention that can test for Zika and two related viruses carried by the same mosquito -- dengue and chikungunya. Hundreds of those test kits are being shipped to labs in affected countries and around the United States.
For vaccines, there are more than 23 projects underway by 14 vaccine developers in the United States, France, Brazil, India and Austria, WHO officials said. Although clinical trials might start before the end of the year, including one underway at the National Institutes of Health, health authorities have said it will be several years at least before a fully tested, licensed Zika vaccine is ready.
Although the "first explosive wave" of Zika's spread may be over before a vaccine is available, Chan said there will continue to be need for a vaccine in the years ahead."We expect it to be a long haul," she said.