The results from a Phase 1 trial into the safety of a landmark chlamydia vaccine have just been published demonstrating promising signs the treatment is safe, and potentially effective in preventing the most common bacterial sexually transmitted infection in the world.

Over 100 million new cases of chlamydia are diagnosed every year, and while it is easily treatable with antibiotics, it can often be symptomless, going undiagnosed for months or even years. As well as causing inflammation, arthritis and ectopic pregnancy, the infection can lead to infertility in women if not diagnosed and treated early.

"The major issue with chlamydia is the long-term consequences," says Robin Shattock, from Imperial College London. "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 program, people get repeatedly re-infected. If you could introduce a protective vaccine, you could break that cycle."

This early human trial is the culmination of over 15 years of research, from first finding a weakness in the chlamydia bacterium, to developing a vaccine that exploits that weakness. As with all Phase 1 human trials, the primary outcome was to evaluate safety. Out of the 30 women in the trial receiving active doses of the vaccine, no serious adverse reactions or complications were noted.

A secondary outcome was also evaluated in this trial, investigating whether the vaccine simulated an immune response in the human subjects. All the subjects receiving the active vaccine displayed significant signs of an immune response, suggesting it should be effective at preventing infections in real-world conditions.

"We took blood samples of the women during the trial," explains Frank Follmann, from Statens Serum Institut (SSI) in Denmark and co-author on the new research. "They showed that all vaccinated women had generated specific antibodies and T cells against chlamydia. Also, as local immunity in the genital tract is important to stop the infection as quickly as possible, during the trial, we collected mucous secretion in a menstrual cup and found high levels of antibodies, including the special mucosal antibody, IgA, which effectively blocks chlamydia early in the course of infection."

All prior research suggests these immune biomarkers indicate the vaccine protects against chlamydia infection, but of course this cannot be completely confirmed without broader trials. The next step for the research is to expand into larger trials with more subjects. So realistically it may be some years before this vaccine makes it into broad human use but Peter Andersen, from SSI's Center for Vaccine Research, already envisions the vaccine easily being combined with the HPV vaccine.

"The HPV vaccine has shown us how effective vaccination can be against a sexually transmitted infection," says Andersen. "We hope to do the same with Chlamydia and, in the long term, combine the two vaccines."

The new trial data was published in the journal The Lancet Infectious Diseases.