An experimental vaccine being developed by U.S. government scientists to prevent the painful mosquito-borne viral disease chikungunya has shown promise in its first human trials but remains years away from approval for widespread use.

In a study published on Thursday in the Lancet medical journal, National Institutes of Health scientists said the vaccine elicited an impressive immune response in all 25 adult volunteers who took part and caused no worrisome side effects.

"We believe it is a highly promising vaccine given how well tolerated it was and how robust the immune responses were," said the leader of the study, Dr. Julie Ledgerwood of the NIH's National Institute of Allergy and Infectious Diseases.

Infection with the virus, spread by two mosquito species, typically is not fatal but can cause debilitating symptoms including fever, headache and severe joint pain lasting weeks or months. There is no current treatment and no licensed vaccine to prevent it.

It showed up for the first time in the Americas late last year. In the United States, locally transmitted infections - as opposed to infections in Americans traveling abroad - have been reported for the first time this year.

The early-stage clinical trial involved 25 healthy American volunteers ages 18 to 50 years old who were given one of three dosage levels of the vaccine in three injections over a 20-week period.

The volunteers were not exposed to the chikungunya virus, but their immune response was measured in the form of neutralizing antibodies - proteins produced by a special type of white blood cell that defends a person from an invading virus.

An immune response was seen in most of the volunteers after the first vaccination. Following the second, all exhibited high levels of antibodies. There also was a significant increase in antibodies after the third injection.

The antibodies lasted a long time and were present in all of the volunteers six months following their final shot.

This was a so-called Phase I trial testing the safety of a vaccine and looking at dosage ranges. Before securing regulatory approval, the vaccine would need to go through a Phase II trial using a larger group of people to test potential effectiveness and further evaluate safety. Next would be a Phase III trial with large numbers of people to confirm effectiveness and safety.

The vaccine already was shown to protect rhesus monkeys from chikungunya.

"A Phase II trial likely would take several more years, both for the production of vaccine as well as development and completion of the trial," Ledgerwood said.

Ledgerwood said the NIH needs to work with private organizations or industry partners to move forward.

"At least one such potential partner is interested," she added without saying who it was. "For most vaccines, development takes decades. We don't think it should take that long for this vaccine."

Vaccines often are made from killed viruses or weakened live viruses. This one is different. It is a virus-like particle (VLP) vaccine similar to the type used against human papillomavirus (HPV).