(Reuters) – Hopes of developing a vaccine against Zika took a small step forward on Wednesday as Inovio Pharmaceuticals Inc said its experimental shot had induced a robust and durable response in mice.

Shares in the U.S. biotech firm, which expects to test its product in humans before the end of the year, jumped 7 percent in premarket trading on the prospect of it developing a vaccine against the mosquito-borne disease.

At least 15 companies and academic groups are currently racing to develop Zika vaccines, according to the World Health Organization (WHO), spurred on by growing public concern over the virus sweeping across the Americas.

Zika, whose symptoms include mild fever and rash, has been linked to brain damage in thousands of babies in Brazil, although the connection is not yet proven.

There is no proven treatment or vaccine for the disease, a close cousin of the viruses that cause dengue, chikungunya and West Nile fever.

Inovio said in a statement that mice given its vaccine showed the development of antibodies and generated a response from T-cells, which play an important role in immunizing the body.

“We will next test the vaccine in non-human primates and initiate clinical product manufacturing. We plan to initiate Phase I human testing of our Zika vaccine before the end of 2016,” Inovio Chief Executive Joseph Kim said.

Phase I is the first stage in a three-step process of testing new medicines and involves giving an experimental product to healthy volunteers.

Inovio’s DNA-based vaccine is being developed with South Korea’s GeneOne Life Sciences Inc and academic collaborators. One Canadian collaborator told Reuters last month that vaccine testing on humans could begin as early as August.

Other organizations with relatively advanced Zika vaccine projects include India’s Bharat Biotech, which said earlier this month that its experimental vaccine would start pre-clinical trials in animals imminently.

The U.S. National Institutes for Health is also working on another DNA vaccine, while France’s Sanofi, which makes the world’s first vaccine for dengue, said on Feb. 2 it was launching a Zika project.

Despite the accelerated work program, however, the WHO estimates it will still be at least 18 months before any Zika vaccines are ready to be tested in large-scale clinical trials.

Much remains unknown about Zika, including whether the virus actually causes microcephaly, a condition marked by an abnormally small head size in newborns, although the WHO believes the suspected link could be confirmed within weeks.

Brazil is investigating the potential link between Zika infections and more than 4,300 suspected microcephaly cases.

An estimated 80 percent of people with Zika have no symptoms, making it difficult for pregnant women to know whether they have been infected.

Experts say the developing of a useable preventative shot against the disease will not be simple, particularly due to concerns surrounding the safety of vaccinating pregnant women.

(Reporting by Ben Hirschler in London and Amrutha Penumudi in Bengaluru; Editing by Kirti Pandey, Greg Mahlich)