Geovane Silva holds his son Gustavo Henrique, who has microcephaly, at the Oswaldo Cruz Hospital in Recife, Brazil.

The World Health Organization on Monday declared the mosquito-borne Zika virus an international public health emergency due to its link to thousands of birth defects in Brazil, as the U.N. agency sought to build a global response to the threat.

WHO Director-General Margaret Chan told reporters that coordinated international action was needed to improve detection and expedite work on a vaccine and better diagnostics for the disease, but said curbs on travel or trade were not necessary.

Chan, whose agency was assailed as too slow in reacting to West Africa’s Ebola epidemic that killed more than 10,000 people in the past two years, cited “first and foremost the big concern about microcephaly,” the birth defect that causes babies to be born with abnormally small heads and improperly developed brains.

She noted that it was “strongly suspected but not yet scientifically proven” that Zika causes microcephaly.

“Can you imagine if we do not do all this work now, and wait until the scientific evidence comes out?” Chan told reporters at the WHO headquarters in Geneva. “Then people will say that, ‘Why don’t you take action because the mosquito is ubiquitous?’”

The emergency designation, recommended by a committee of independent experts following criticism of a hesitant response to Zika so far, should help fast-track international action and research priorities. The move lends official urgency to research funding and other steps to stem the spread of the virus.

The WHO said last week the Zika virus, transmitted by the bite of the Aedes aegypti mosquito, was “spreading explosively” and could infect as many as 4 million people in the Americas. The Pan American Health Organization says Zika has spread in 24 nations and territories in the Americas.

This marks the fourth time the WHO has declared a global health emergency since such procedures were put in place in 2007, with the others arising from influenza, Ebola and polio.