 This non-randomized, multicenter, open-label study will assess the safety and efficacy of subcutaneously administered trastuzumab in participants with early and locally advanced HER2-positive breast cancer in two sequential cohorts. First 120 participants will be treated with subcutaneous SC trastuzumab 600 milligrams mg vial Cohort A and the subsequent 120 participants will be treated with SC trastuzumab prefilled single use injection device SID Cohort B. Participants from each cohort will receive neoadjuvant or adjuvant chemotherapy consisting of doxorubicin every 3 weeks q3w 1 cycle for 4 cycles followed by paclitaxel weekly or docetaxel every 3 weeks q3w in combination with SC trastuzumab 600 mg q3w for 4 cycles and a further 14 cycles of SC trastuzumab 600 mg q3w alone. All participants will be followed up for 24 months after the last participant has received the last dose of study treatment, or earlier in case of withdrawal from the study, loss to follow-up or death. 