WASHINGTON (Reuters) - A dengue vaccine developed by U.S. National Institutes of Health scientists protected everyone given the shot against the virus in a promising small study published on Wednesday, with the researchers saying it could become widely available by 2018.

The scientists also expressed optimism that the approach they used for the dengue vaccine could work in creating a vaccine against the Zika virus, which is in the same viral family and spread by the same mosquito species. Zika, linked to numerous cases of the birth defect microcephaly in Brazil, is spreading rapidly in Latin America and the Caribbean.

Researchers administered the single-dose vaccine, called TV003, to a group of volunteers and six months later exposed them to dengue-2, one of the four different strains of the virus.

All 21 people given the vaccine were protected from infection. All 20 people given a placebo injection developed dengue infection after being exposed to the virus. Everyone in the placebo group had the virus in their blood, 80 percent developed a rash and 20 percent exhibited low white blood cell counts.

The results were very promising and inspired "great confidence" that the vaccine will protect people in areas where dengue is endemic, said vaccine researcher Dr. Anna Durbin of the Johns Hopkins Bloomberg School of Public Health in Baltimore.

Dengue, found in the world's tropical and subtropical regions, infects nearly 400 million people in more than 120 countries annually. Most survive with few or no symptoms, but more than 2 million people annually develop dengue hemorrhagic fever, which kills more than 25,000 people each year.

"Control of dengue has certainly been a public health priority for many years, but getting there hasn't really been very easy," said virologist Stephen Whitehead of the NIH's National Institute of Allergy and Infectious Diseases, who spearheaded development of the vaccine.

The vaccine was made from a mixture of four live, weakened viruses targeted to each of the four different strains.

The volunteers were exposed to a genetically modified version of dengue-2 virus isolated in Tonga in 1974 that was known for causing only mild illness.

Based in part on this study, in February Brazil's Butantan Institute launched a large Phase III clinical trial to confirm the effectiveness of the vaccine against naturally occurring dengue, with 17,000 people due to take part, Durbin said.

Another trial in Bangladesh is scheduled to begin in the next couple of months, Durbin added.

If the trial in Brazil goes well, Butantan Institute could have the vaccine widely available by 2018, Durbin said.

One dengue vaccine is currently licensed, Sanofi SA's Dengvaxia, with Mexico in December becoming the first country to give it approval. But the three-dose vaccine was approved only for use in a limited population, people ages 9 to 45 who live in areas where the disease is endemic, meaning younger children and tourists could not get it, and questions remain about its effectiveness.

Whitehead said Merck and Co has exclusive rights to the new vaccine in the United States, Canada, China, Japan and the EU and can export it to any country except Brazil, where the Butantan Institute has exclusive rights.

Two Indian companies, Serum Institute of India and Panacea Biotec Ltd, have non-exclusive rights to develop the vaccine for India and for export to other countries except where Merck and Butantan have exclusive rights, Whitehead said. A Vietnamese company, Vabiotech, has a non-exclusive license to produce it locally in Vietnam and for export to other nations except where Merck and Butantan have exclusive rights, he added.

The research was published in the journal Science Translational Medicine.

(Reporting by Will Dunham; Editing by Michele Gershberg and Chizu Nomiyama)