A Vaccine (H2NVAC) Before Surgery for the Treatment of HER2-Expressing Ductal Carcinoma In Situ

This study is testing a vaccine called H2NVAC before surgery in people with ductal carcinoma in situ (DCIS) — a very early stage of breast cancer that has not spread — where the cancer cells have a protein called HER2 on their surface. The goal is to train the immune system to attack HER2-expressing cancer cells. **You may be eligible if...** - You have confirmed DCIS (non-invasive breast cancer) that has not yet been treated or surgically removed - Your tumor cells show HER2 expression at any level (1+, 2+, or 3+) - You have not previously received treatment for this DCIS - You are in good general health (ECOG performance status ≤ 2) - Your blood counts and organ function are within acceptable levels - You agree to use effective contraception from enrollment through 6 months after the final vaccination **You may NOT be eligible if...** - You are pregnant, breastfeeding, or unwilling to use contraception - You have an immunodeficiency condition or are on chronic steroids - You have HIV or an active autoimmune disease - You have another active cancer within the past 3 years - You have had prior radiation to the affected breast - Your heart function (LVEF) is below 55% - You have a known allergy to GM-CSF (an ingredient in the vaccine) Talk to your doctor to see if this trial is right for you.