Chlamydia is the most common sexually transmitted infection (STI) in the world, with almost 100 million new cases each year.
Sometimes symptomless, it can easily be missed. In women especially this can lead to infertility if not treated quickly enough.
This vaccine would easy affect millions of people's lives around the world, and potentially minimize the spreading of the STI, much like the HPV vaccine.
Teams of researchers from Denmark's Statens Serum Institute (SSI) and the U.K.'s Imperial College London have been working on the trial for years.
The study was published in the scientific journal The Lancet Infectious Diseases.
"The vaccine showed the exact immune response we had hoped for and which we have seen in our animal tests," said Frank Follman, head of department at SSI.
Follman continued, "The most important result is that we have seen protective antibodies against chlamydia in the genital tracts. Our initial trials show them preventing the chlamydia bacteria from penetrating the cells in the body."
"This means we have come a lot closer to a vaccine against chlamydia," finished Follman.
The team do highlight, though, that even though this is a very promising first step, more trials are now needed to determine whether or not the immune response provoked by the vaccine will properly protect against chlamydia infection in the real world .
What is chlamydia?
As the most prevalent STI on Earth, it's a very common infection, with three out of four infections showing up as symptomless. This makes it a hard one to catch.
Unfortunately, screening programs and antibiotic treatments so far have not been able to minimize infection rates.
Currently, there are treatments for chlamydia, however, some complications can occur, leading to infertility in women, inflammation, ectopic pregnancy, and a higher susceptibility to other STIs, including HIV.
Professor Robin Shattock from Imperial College London, said "One of the problems we see with the current efforts to treat chlamydia is that despite a very big screening, test and treat program, people get repeatedly re-infected."
Shattock continued "If you could introduce a protective vaccine, you could break that cycle."
There's still work to be done, but the first results are promising.