A vaccine has for the first time been shown to protect against the sexually transmitted infection gonorrhoea, scientists in New Zealand say.
There are fears gonorrhoea is becoming untreatable as antibiotics fail.
The World Health Organization sees developing a vaccine as vital in stopping the global spread of "super-gonorrhoea".
The study of 15,000 young people, published in the Lancet, showed infections were cut by about a third.
About 78 million people pick up the sexually transmitted infection each year, and it can cause infertility.
But the body does not build up resistance no matter how many times someone is infected.
The vaccine, originally developed to stop an outbreak of meningitis B, was given to about a million adolescents in New Zealand between 2004 and 2006.
Researchers at the University of Auckland analysed data from sexual health clinics and found gonorrhoea cases had fallen 31% in those vaccinated.
The bacterium that causes meningitis, Neisseria meningitidis, is a very close relative of the species that causes gonorrhoea - Neisseria gonorrhoeae.
It appears the Men B jab was giving "cross-protection" against gonorrhoea.
Dr Helen Petousis-Harris, one of the researchers, said: "This is the first time a vaccine has shown any protection against gonorrhoea.
"At the moment, the mechanism behind this immune response is unknown, but our findings could inform future vaccine development."
Protection seemed to last about two years.
What is gonorrhoea?
The disease is caused by the bacterium Neisseria gonorrhoeae and spread by unprotected sex.
Symptoms can include a thick green or yellow discharge from sexual organs, pain when urinating and bleeding between periods.
However, of those infected, about one in 10 heterosexual men and more than three-quarters of women and gay men have no easily recognisable symptoms.
Untreated infection can lead to infertility, pelvic inflammatory disease and be passed on to a child during pregnancy.
However, the vaccine in question - known as MeNZB - is no longer available.
Many of its components are also in a new Men B jab - called 4CMenB.
The UK is the only country in the world to be rolling 4CMenB out as a routine childhood immunisation.
Fellow researcher Prof Steven Black, from Cincinnati Children's Hospital in the US, said: "The potential ability of a group B meningococcal vaccine to provide even moderate protection against gonorrhoea would have substantial public health benefits."
The importance of preventing people developing a gonorrhoea infection is of mounting importance as the infection is getting much harder to treat.
Last week, the World Health Organization warned about the global spread of gonorrhoea that could not be treated with antibiotics.
Dr Teodora Wi, from the WHO, said there had even been three cases - in Japan, France and Spain - where the infection was completely untreatable.
She said: "There are high hopes that now there's going to be some cross-protection.
"We are still a long way before we develop a vaccine for gonorrhoea, but we have now some evidence that it is possible."
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