This study investigates the safety of an investigational vaccine in patients with HER2 positive breast cancer. HER2 positive breast cancer is a type of breast cancer that tests positive for a protein called human epidermal growth factor receptor 2, which promotes the growth of cancer cells. The study also looks at how the vaccine affects the time it takes for the cancer to come back, known as disease-free survival.

Participants in the study will receive either the investigational vaccine or a placebo, which is an inactive substance that looks like the investigational vaccine but does not contain any medicine. The study will monitor the health and progress of participants to evaluate the vaccine's safety and its effect on disease recurrence.

• Who can participate: Participants must have stage I-III HER2 positive breast cancer and have completed neoadjuvant chemotherapy. They should have normal organ function and agree to use effective contraception.
• Study details: Participants will be randomly assigned to receive either the investigational vaccine or a placebo. The study will monitor health changes and disease recurrence.