A sharp increase in microcephaly cases in Brazil has triggered a global health emergency over the mosquito-borne virus, which had previously been viewed as causing only a relatively mild illness, and spurred a race to develop a vaccine, medicines and better diagnostic tests.
The WHO said U.S. government scientists and an Indian biotechnology firm were the front-runners in the vaccine effort but said it would take at least 18 months to start large-scale clinical trials of potential preventative shots. The U.N. health agency also for the first time advised pregnant women to consider delaying travel to Zika-affected areas.
Brazil is at the center of the Zika outbreak that has spread to more than 30 countries. Researchers there are working to determine whether Zika has caused a big rise in cases of microcephaly, a birth defect in which babies are born with abnormally small heads and may have developmental problems.
Brazil's health ministry issued fresh figures on February 12, reporting 4,314 suspected and confirmed cases of microcephaly, up from 4,074 cases on Feb. 2. The ministry said it had confirmed 462 of those cases as microcephaly or other alterations to the central nervous system. Researchers have identified evidence of Zika infection in 41 of these cases, either in the baby or in the mother. But scientists have not confirmed that Zika can cause microcephaly.
"It seems indeed that the link with Zika (and microcephaly) is becoming more and more probable, so I think that we need a few more weeks and a few more studies to have this straight," Marie-Paule Kieny, WHO assistant director-general for health systems and innovation, told a news briefing in Geneva.
Studies of Zika-infected pregnant Latin American women who were due to deliver their babies soon should yield evidence, Kieny said, adding that data also was coming from studies in French Polynesia and Cape Verde.
Kieny said Zika-hit areas also have experienced increased cases of the neurological disease Guillain-Barre, adding: "The direct causality has still to be demonstrated but the association in time and in location seems to be clear."
Guillain-Barre syndrome, in which the body's immune system attacks part of the nervous system, causes gradual weakness in the legs, arms and upper body and sometimes total paralysis.