A vaccination against chlamydia is safe and has shown signs that it can work, a clinical trial has found.
When tested, the vaccine provoked an immune response on 35 healthy women, researchers have found.
More trials are needed to see whether it can fully protect against the sexually transmitted infection but progress so far is an "important first step", experts say.
The study tested two different formulations of the vaccine and, while both provoked an immune response, one performed better than the other and produced more antibodies.
The participants given a placebo showed no immune response.
Professor Robin Shattock, from Imperial College London, said: "The findings are encouraging as they show the vaccine is safe and produces the type of immune response that could potentially protect against chlamydia.
"The next step is to take the vaccine forward to further trials but, until that's done, we won't know whether it is truly protective or not."
Chlamydia, the world's most common bacterial STI, accounts for almost half of STIs diagnosed in England.
Chlamydia can lead to infertility and complications including ectopic pregnancy and arthritis. It also leaves people more susceptible to other STIs such as HIV.
In most cases, there are no symptoms and, while the infection is usually treated with antibiotics, people can still catch it again.
Professor Shattock said: "It is very treatable if identified, but as many people don't have symptoms it can be missed, and the biggest problem is that it can go on to cause infertility in women.
"One of the problems we see with current efforts to treat chlamydia is that despite a very big screening, test and treat programme, people get repeatedly re-infected. If you could introduce a protective vaccine, you could break that cycle."
The study was led by Imperial College London and the Statens Serum Institut in Copenhagen and their research was published in the journal The Lancet Infectious Diseases.
They also found that the vaccines did not cause any serious side effects.