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.