(Reuters) – Companies and scientists are racing to create a Zika vaccine as concern grows over the mosquito-borne virus that has been linked to severe birth defects and is spreading quickly through the Americas.

Zika is now present in 23 countries and territories in the Americas. Brazil, the hardest-hit country, has reported around 3,700 cases of the devastating birth defect called microcephaly that are strongly suspected to be related to Zika.

The Geneva-based World Health Organization (WHO), stung by criticism that it reacted too slowly to West Africa’s Ebola epidemic, is convening an emergency meeting on Monday to help determine its response to the spread of the virus.

The U.S. Centers for Disease Control and Prevention has activated an emergency operations center staffed around the clock to address Zika, agency officials told Reuters.

On Thursday, the WHO forecast that as many as 4 million people in the Americas may become infected by Zika, lending new urgency to research efforts already under way. Vaccine developers made clear that a vaccine for widespread public use is at least months, if not years, away.

The closest prospect may be from a consortium including drugmaker Inovio Pharmaceuticals Inc that could have a vaccine ready for emergency use before year-end, according to one of its lead developers. Inovio’s share price jumped as much as 13 percent on Friday.

Canadian scientist Gary Kobinger told Reuters on Thursday the first stage of testing on humans could begin as early as August. If successful, the vaccine might be used during a public health emergency by October or November, said Kobinger, who helped develop a trial vaccine for the Ebola virus.

Privately owned vaccine developer Hawaii Biotech Inc said it began a formal program to test a Zika vaccine last fall as the virus started to gain traction in Brazil, although it has no timetable yet for clinical trials.

“Right now, we are in the pre-clinical stage, as I suspect everyone is,” Chief Executive Dr. Elliot Parks told Reuters.

Another private vaccine developer, Boston-based Replikins Ltd, said it was preparing to start animal studies on a Zika vaccine in the next 10 days. Data from the trials on mice and rabbits would likely be out in the next couple of months, Replikins Chairman Samuel Bogoch told Reuters.

“No one has the $500 million on hand to take it (a vaccine) all the way to human trials. Somewhere along the course we hope to have big pockets join us,” Bogoch said.

‘FIGHT THE MOSQUITO’

Zika had been viewed as a relatively mild illness until Brazilian health officials identified it as a matter of concern for pregnant women. While a direct causal relationship has not been established, scientists strongly suspect a link between Zika and thousands of children born in Brazil with abnormally small heads, brain defects and impaired vision.

There is no treatment for Zika infection. About 80 percent of those infected experience no symptoms, making it difficult for pregnant women to know whether they have been infected.

Efforts to combat Zika are focused on protecting people from being bitten and on eradicating mosquitoes, a tough task in many parts of Latin America, where people live in poverty and there are plentiful breeding grounds for the insect.

“We do not have a vaccine for Zika yet. The only thing we can do is fight the mosquito,” Brazilian President Dilma Rousseff said on Friday, reiterating her call for a national eradication effort.

Rousseff said tests for the development of a vaccine would begin next week at the Butantan Institute, one of Brazil’s leading biomedical research centers in Sao Paulo.

Zika has hit Brazil just as it prepares to host the Olympic Games in Rio de Janeiro on Aug. 5-21, an event that draws hundreds of thousands of athletes, team officials and spectators. The International Olympic Committee (IOC) assured teams on Friday the Olympics would be safe from Zika, but urged visitors to carefully protect themselves.

U.S. lawmakers have begun to press the Obama administration for details of its response to Zika. At least 31 people in the country have been infected, all of them after travel to affected countries.

“We need to ensure that federal agencies are working closely together and with the international community to stop its rapid spread,” said Republican Ron Johnson, chairman of the U.S. Senate Homeland Security and Governmental Affairs Committee.

The United States has two potential vaccine candidates and may begin human clinical trials by the end of 2016, U.S. officials said, but there will not be a widely available vaccine for several years.

(Reporting by Natalie Grover and Amrutha Penumudi in Bengaluru; Additional reporting by Julie Steenhuysen in Chicago, Karolos Grohmann in Berlin, Anthony Boadle in Brasilia, Jeffrey Dastin in New York; Writing by Frances Kerry; Editing by Will Dunham)