Brazil, worst hit by the Zika outbreak, said on Wednesday that most of the 508 confirmed cases of microcephaly reported in the country are likely related to the virus, and called its previous count too conservative. Brazil is investigating more than 3,900 additional suspected cases of microcephaly, but it has not yet been proven that the virus can cause the rare defect.

Microcephaly is marked by abnormally small head size that can result in developmental problems. The Health Ministry reported on Wednesday a total of 4,443 suspected and confirmed cases of microcephaly, up from 4,314 a week earlier.

Researchers on Wednesday reported finding the virus in the amniotic fluid of two fetuses diagnosed with microcephaly, suggesting Zika virus can cross the placental barrier and providing another piece of evidence linking the virus with the birth defect.

There is no treatment for Zika, which had been viewed as a relatively mild illness until the concerns over microcephaly and Guillain-Barre syndrome emerged.

At least 15 companies and academic groups are rushing to develop a vaccine. Hopes of a breakthrough took a small step forward on Wednesday when US biotech firm Inovio Pharmaceuticals Inc said its experimental shot had induced a robust and durable response in mice.

Shares of the US biotech firm, which expects to test its product in humans before the end of the year, rose by as much as 7 percent on Wednesday.