Fifty years ago, researchers tested potential vaccines against Chlamydia in both adults and children, injected through the skin. Some subjects developed became part of a strange trend. Instead of teaching the human body to fight the bacteria (Chlamydia or Chlamydophila sp. ) that causes the disease, people became more susceptible to infection. The researchers couldn’t figure out why these patients had such an unexpected outcome, and they dropped the work. Now, new research published June 18 in Science hints at what went wrong, Arielle Duhaime-Ross reports for The Verge.

The team behind the research has also come up with a promising vaccine of their own against the STD. Which is why they had to tackle the long, checkered history of vaccines for Chlamydia. "It will be very hard to convince anyone to try your vaccine unless you can explain why there might have been this paradoxical effect 50 years ago and why we are confident that this paradoxical effect will not be observed with the current formulation," Ulrich von Andrian, an immunologist at Harvard and a co-author on the study, said in a statement. "I think we can provide reasonable answers to both of these questions."

To figure out what went wrong the first time, a team of immunologists from Harvard, MIT and Brigham and Women's Hospital in Boston tried to recreate the 1960s studies; this time in mice, not humans. They inoculated some mice with a live version of the bacteria and some with a dead version of the bacteria, similar to what might be used in a vaccine. Then they gave all the mice a second dose of the live virus. Mice that had been injected with the dead vaccine were more likely to get sick.

Armed with that knowledge, the team developed their own version, a vaccine that delivered the live vaccine via nanoparticles. In mice, it seems to work, but the vaccine still hasn't been tested on any humans. Today, Chlamydia infects about 100 million people each year. It can be treated pretty easily, but since the 1960s, Chlamydia vaccine research has largely stalled, due to the confusing results. This new research could pave the way towards a vaccine candidate that’s both safe and effective.