United States health authorities Wednesday launched the second human trial of a vaccine against the mosquito-borne Zika virus, which can cause birth defects and is now spreading in the US and Latin America.
The National Institute of Allergy and Infectious Diseases (NIAID) Zika virus investigational DNA vaccine will be given to 80 healthy volunteers aged 18-35 at three study sites in the United States, said the federal agency.
The first dose was delivered Tuesday, the NIAID said in a statement.
"A safe and effective vaccine to prevent Zika virus infection and the devastating birth defects it causes is a public health imperative," said NIAID director Anthony S Fauci.
"Results in animal testing have been very encouraging. We are pleased that we are now able to proceed with this initial study in people."
Fauci, who has previously warned that any vaccine against Zika would take years to develop, described the trial's launch as "an important step forward."
The drug is a DNA vaccine that is similar to another experimental vaccine developed by NIAID against West Nile virus.
It cannot infect people with Zika, but is designed to make the body mount an immune response to Zika.
DNA vaccines "have been shown to be safe in previous clinical trials for other diseases," the NIAID statement said.
The first Zika vaccine trial got under way last month.
Led by Pennsylvania-based Inovio Pharmaceuticals, the phase 1 trial using a Zika DNA vaccine called GLS-5700 is being conducted in Quebec City, Canada, as well as Miami and Philadelphia.
The US Centres for Disease Control and Prevention say more than 50 countries and territories, mainly in Latin America and the Caribbean, currently have active transmission of Zika virus.
The United States last month announced its first locally transmitted cases of Zika in Miami, with more than a dozen infections among non-travelers.
The CDC has issued a travel warning for a square-mile area where the Zika-infected mosquitoes are believed to be lurking.
There is no cure for Zika. In four out of five cases, it causes no symptoms. People who do feel sick from it may experience a rash, joint pain or eye infection.
However, in pregnant women Zika can cause permanent damage to the developing fetus, including microcephaly, a condition in which the infant's skull and brain are unusually small.
Women who are pregnant or want to become pregnant are urged to avoid areas where Zika is circulating, and take steps to prevent mosquito bites.
Barrier methods of contraception are also recommended, since the virus can be spread by sexual contact.
Initial results from the NIAID vaccine trial are expected in January 2017. If it appears safe, a phase 2 trial in Zika-endemic countries could begin soon after.
A study out last month in the journal Science projected that the Zika pandemic would fizzle out in the next two to three years.
Zika cannot infect the same person twice, so once a large number of people build up immunity to it, the surge in cases will subside, argued lead author Neil Ferguson, a professor at Imperial College London's School of Public Health.
"This means by the time we have vaccines ready to be tested, there may not be enough cases of Zika in the community to test if the vaccine works," he said.