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The study will involve more than 900 women worldwide and aims to show if the disease can be controlled by a simple monthly injection.
It will initially involve women with the disease in its advanced stages.
Standard treatments
When a woman is diagnosed as having breast cancer, doctors will first surgically remove any lumps.
They must then destroy any remaining cells using chemotherapy, a powerful cocktail of anti-cancer drugs.
However, chemotherapy has unpleasant side-effects and can be ineffective.
Dr David Miles, a consultant at Guy's and St Thomas's hospital, London, is co-ordinating the trial of the new vaccine.
He said: "Advanced breast cancer can be difficult to treat. The main aim is often to control the disease while allowing the patient to live as normal a life as possible.
"This treatment appears to have few side-effects, can be easily applied once a month by injection, and allows patients to live their lives with what we hope will be minimal discomfort."
But Dr Miles said that even if the trial proves a success, the vaccine is unlikely to replace chemotherapy.
It would be used alongside the current treatments.
Immune response
The vaccine is thought to work by using the body's immune system.
Normally the immune system does not recognise cancerous cells as a threat, and so they are left to grow unhindered.
The vaccine is thought to work by "marking" cancers.
Normal cells have a protective coating of sugars around their core. In breast cancer cells, much of the coating is missing.
This means that sugars closer to the core are exposed.
Cancer cells destroyed
The vaccine targets one of these sugars, known as STn.
Antibodies in the immune system can then home in on the cancer cells and destroy them.
Early studies have shown promising results.
Dr Miles said: "Data from the studies suggest that the vaccine could help keep the disease at bay and prolong patients' lives.
"But the only way to be sure is by doing a much bigger study."
He said it would take four to five years before researchers knew the real value of the vaccine.
Background Briefings
Medical notes
Ginger Spice: 'I feared mastectomy'
(28 Sep 98 | Health)
Breast cancer factfile
(23 Sep 98 | Medical notes)
Chemotherapy that beats breast cancer
(17 Sep 98 | Health)
Tamoxifen could be first cancer prevention treatment
(07 Aug 98 | Health)
Imperial Cancer Research Fund
Breakthrough Breast Cancer
Guy's and St Thomas' Hospital Trust
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