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Many people who were vaccinated against the New Zealand meningococcal epidemic may have been unexpectedly protected against a sexually-transmitted disease.
Scientific detective work has shown that people who received the vaccine are 30% less likely to be diagnosed with gonorrhoea than those who didn't receive it.
The University of Auckland finding has sparked international interest among sexual health and vaccine experts because of the desperate need for the world's first vaccine against gonorrhoea, owing to disease's growing resistance to antibiotics. There are fears it will eventually be untreatable.
"This is an exciting discovery that has the potential to help combat a very common and distressing disease,'' said the lead researcher, Dr Helen Petousis-Harris.
The MeNZ-B vaccine was developed specifically to target the New Zealand B-strain of meningococcal disease, which had caused an epidemic since 1991. The vaccine was offered free to all under age 20 between 2004 and 2006, by which time the epidemic was waning. Vaccination ended in 2008 for babies and infants, and for people with high-risk medical conditions in 2011.
Gonorrhoea is reported in more than 3000 people a year in New Zealand, although many cases aren't reported. The number of cases had been rising steadily, but researchers noticed a sustained reduction from 2006 to 2011/12 and began to question whether there might be a link to the MeNZ-B programme. Similar effects had been noticed in Cuba and Norway after they began meningococcal-B vaccination programmes.
The bacteria which cause meningococcal disease have genetic links to those that cause gonorrhoea. A link was plausible because the two bacteria - Neisseria meningitidis and Neisseria gonorrhoeae - have 80 to 90% of their primary DNA sequences in common.
"Scientists thought it wouldn't work, so it's taken us all by surprise,'' said sexual health physician Dr Jane Morgan.
"It's a dreadful disease. Left untreated, gonorrhoea can cause severe pain and reduced fertility for women.''
She said the growing antibiotic resistance of gonorrhoea was causing international alarm. "This makes the development of a gonorrhoea vaccine an essential component in reducing the burden of this nasty disease.''
Although the New Zealand vaccine is no longer available, Britain is giving a meningococcal B vaccine, 4CMenB, which contains the key MeNZ-B ingredient, to babies.
Dr Morgan said discussions were under way about doing a high-quality clinical trial of that vaccine in New Zealand to see if it prevents gonorrhoea. The logical place to run the trial would be in the Gisborne region and others with high rates of the disease.
Reflecting its relatively high socio-economic deprivation, Gisborne has New Zealand's highest rates of gonorrhoea, at six times the national average for females, and three times for males.