Of 14 women with advanced breast cancer who received the vaccine, half showed no sign of tumour growth one year after treatment.
The vaccine had an effect even in those with immune systems weakened by the disease and chemotherapy. Scientists now plan to follow the small pilot trial with a larger study of newly diagnosed patients who should have stronger immune systems.
US lead researcher Professor William Gillanders, from Washington University School of Medicine, said: “Despite the weakened immune systems in these patients, we did observe a biologic response to the vaccine while analysing immune cells in their blood samples.”
The vaccine primes the immune system to target a protein called mammaglobin-A which is found almost exclusively in breast tissue. Breast cancer tumours produce it at abnormally high levels.
“Being able to target mammaglobin is exciting because it is expressed broadly in up to 80 per cent of breast cancers, but not at meaningful levels in other tissues,” said Prof Gillanders.
“In theory, this means we could treat a large number of breast cancer patients with potentially fewer side effects.”
The larger trial is expected to provide more meaningful results, said Prof Gillanders.
Sally Greenbrook from the charity Breakthrough Breast Cancer said: “These are very interesting results, and this work could eventually lead to a therapeutic treatment or a vaccine that could be used in breast cancer prevention.
“However it is important to remember that this was a very small-scale trial designed to see whether the treatment is safe and effective, so any vaccine against breast cancer is likely still some years away.
“As treatments and clinical preventative measures improve it is equally important that women manage their own risk of breast cancer by maintaining a healthy lifestyle.”