This clinical trial is focused on studying HER2-positive breast cancer, 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 is evaluating the effectiveness and safety of two treatments: atezolizumab and trastuzumab emtansine. Atezolizumab, also known by its code name RO5541267, is a medication that helps the immune system fight cancer. Trastuzumab emtansine is a combination of two drugs that work together to target and kill cancer cells. The study also includes a comparison with a placebo, which is a substance with no active medication.

The purpose of the study is to determine how well atezolizumab works when given with trastuzumab emtansine, compared to trastuzumab emtansine alone. Participants in the study will receive one of these treatments after they have completed their initial therapy for breast cancer. The study will monitor participants over a period of time to assess their health and any changes in their condition. This will help researchers understand the potential benefits and risks of the treatments.

Participants will be randomly assigned to receive either the combination of atezolizumab and trastuzumab emtansine, trastuzumab emtansine with a placebo, or other variations as part of the study. The treatments will be administered through an infusion, which is a method of delivering medication directly into the bloodstream. The study aims to provide valuable information on the effectiveness of these treatments in preventing the recurrence of breast cancer in patients who are at high risk.