An experimental breast cancer vaccine has been shown to slow progression of the disease in human patients.
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.
"That's very encouraging. We also saw preliminary evidence of improved outcome, with modestly longer progression-free survival."
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% 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 vaccine would not be effective in the small number of breast cancer patients whose tumours do not generate mammaglobin-A, he pointed out.
The small Phase I trial was chiefly designed to assess the vaccine's safety and side effects, but the scientists also obtained preliminary data on its effectiveness.
The 14 treated patients all had metastatic, or spreading, breast cancer and tested positive for mammaglobin-A.
About one fifth of a comparison group of 12 women who were not vaccinated also showed no cancer progression after one year.
The better outcome seen in the vaccinated group was statistically significant, said the scientists whose findings are reported in the journal Clinical Cancer Research.
The larger trial is expected to provide more meaningful results, said Prof Gillanders.
He added: "If we give the vaccine to patients at the beginning of treatment, the immune systems should not be compromised like in patients with metastatic disease.
"We also will be able to do more informative immune monitoring than we did in this preliminary trial.
"Now that we have good evidence that the vaccine is safe, we think testing it in newly diagnosed patients will give us a better idea of the effectiveness of the therapy."
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."