Of 14 women with advanced breast cancer who received the drug, half showed no sign of tumour growth a year after treatment.
The vaccine had an effect even in those with immune systems weakened by the disease and chemotherapy.
Scientists plan to follow the trial with a larger study of newly diagnosed patients, who may have stronger immune systems.
US lead researcher Professor William Gillanders, from Washington University School of Medicine, said: “Despite the weakened immune systems, 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 mammaglobin-A, a protein found almost exclusively in breast tissue.
Breast cancer tumours produce it at abnormally high levels.
Prof Gillanders said: “Being able to target mammaglobin is exciting.
“It is expressed broadly in up to 80 per cent of breast cancers, but not at meaningful levels in other tissues.
“In theory, this means we could treat a large number of patients with potentially fewer side-effects.”
The scientists, whose findings are reported in the journal Clinical Cancer Research, said the vaccine would not be effective in cancer patients with tumours that do not generate mammaglobin-A.
Sally Greenbrook, from Breakthrough Breast Cancer, said: “These are very interesting results.”