The first chlamydia vaccine to be tested in humans has been shown to be safe, and may protect against the most common sexually transmitted bacterial infection in the world.

Research teams in Denmark and the United Kingdom reported findings Monday from their phase one clinical trial in The Lancet Infectious Diseases.

"Of course, the research is still in its early days," said study author Frank Follmann, director of the department of infectious disease immunology at Statens Serum Institut in Denmark. "But we're very happy. We found a robust response."

His team tested the vaccine in 35 women who were not infected with chlamydia. Some received a placebo.

Every single woman who got the vaccine showed an immune response, compared to none of the women who got the placebo shot.

"We hope this will be the first of many trials, so we can measure efficacy in the real world," Follmann told NBC News.

It does not yet prove the vaccine will be effective in preventing transmissions. Right now, the best method of prevention is the proper use of condoms.

Infectious disease experts say the experimental vaccine is promising because it seems to target the bacterial infection where it hides: inside the cells.

"A lot of the current vaccines that are used in people induce mostly an antibody response," said Dr. Toni Darville, chief of pediatric infectious disease at the University of North Carolina Children's Research Institute, who was not involved with the study. "The problem with chlamydia is that it lives inside cells and replicates inside cells. You really need a T-cell response to combat this infection."

Darville also wrote a commentary accompanying the newly published research.

There were more than 1.7 million cases of chlamydia in the United States in 2017 alone, according to the Centers for Disease Control and Prevention. Most of the 200,000 cases diagnosed each year occur in young people.

"Up to 10 percent of sexually active teenagers and young adults are infected with chlamydia," Darville said.

How does chlamydia hide?

The infection is treated easily with antibiotics, but many people may go years without treatment because the disease lurks undetected within the body without causing symptoms in about three-quarters of those who become infected.

"Chlamydia is an expert in hiding," Follmann said. "People go around with chlamydia infections without knowing it and spread the infections to other partners." It's only discovered through screening, when a young woman goes to a doctor for birth control or other treatment.

This is why a vaccine given well in advance of any possible transmission would be a boon to public health, and women's health in particular.

In about 1 out of every 6 cases, the infection travels up a woman's cervix, making it a leading cause of sexually transmitted pelvic inflammatory disease. Over time, it can scar the fallopian tubes, increase the risk for ectopic pregnancies or cause infertility. And emerging evidence suggests repeated chlamydia infections may be linked to ovarian cancer.

"It’s a small subset of women who develop these long-term complications, but when you have millions of people infected, those numbers add up," Darville said.

“You might get this when you’re 16 or 18, and then when you’re in your late 20s or 30s, you’re infertile and you don’t know why. And it’s because you had chlamydia years ago," Darville told NBC News.

The experimental chlamydia vaccine has a long way to go before it's available. It must be tested in many more people and for a longer period of time to assess its safety and effectiveness.

"My hope would be to combine it with the HPV vaccine and deliver it in the same manner, at the same time," Darville said. The HPV vaccine targets the strains of the human papilloma virus responsible for the majority of cervical cancers and genital warts.

"The HPV vaccine prevents cancer," Darville said. "This vaccine would prevent infertility."

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