Participants will receive Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, IV Herceptin will be given, and during Cycles 5 to 8, SC Herceptin will be administered via handheld syringe using the vial formulation. In the continuation period, participants will receive SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration will be performed by HCP throughout the study.

Herceptin will be given on Day 1 of each 3-week cycle for a total of 18 cycles. If study treatment for Cycle 1 is IV Herceptin, the initial dose will be a loading dose of 8 mg/kg for de novo participants who start Herceptin treatment in the study. For all other cycles where IV Herceptin is given and for non-de novo participants, the dose will be 6 mg/kg. The SC dose will be 600 mg for both the SID and vial formulations for all cycles where SC Herceptin is given.

The SID will be used, containing Herceptin 600 mg per 5 milliliters (mL).

The SID will be used, containing Herceptin 600 mg per 5 mL.

The time required to perform each method of drug administration was assessed via questionnaire with each HCP by asking to rate the amount of time it took to administer each method of drug administration (IV or SC Herceptin) at the end of the crossover period (Week 24). Time was rated in the following time block categories: less than (<) 5 minutes, 6 to 10 minutes, 11 to 15 minutes, 16 to 20 minutes, and greater than (>) 20 minutes. Responses of "Not Sure" and "Unknown" were also allowed. The percentage of HCPs who rated the amount of time in each of the categories was reported.

EFS events included local, regional, or distant recurrence of the original breast cancer, occurrence of contralateral breast cancer, or death due to any cause. The percentage of participants who had an EFS event at any time on study was reported.

EFS was defined as the time from randomization to a local, regional, or distant recurrence of the original breast cancer, occurrence of contralateral breast cancer, or death due to any cause. The median duration of EFS and corresponding 95 percent (%) CI according to Kaplan-Meier estimates were planned to be reported and expressed in months.

EFS events included local, regional, or distant recurrence of the original breast cancer, occurrence of contralateral breast cancer, or death due to any cause. The proportion of participants without an EFS event (i.e., the EFS rate) and corresponding 95% CI at 3 years after randomization was reported.

Participants who performed self-administration of SC Herceptin via SID were given an evaluation questionnaire during the continuation period (Weeks 25 to 52) after their first self-administration. Participants responded to 5 statements about their comfort with self-injection, the convenience of the SID, their self-confidence using the SID, their satisfaction with the SID, and whether they would consider using the SID again in the future. Each statement used a 5-item rating scale with responses from "Strongly Disagree" to "Strongly Agree". The percentage of participants with a positive response (either "Agree" or "Strongly Agree") to each questionnaire statement was reported.

Participants in Cohort 1 provided blood samples for immunogenicity testing to assess for anti-drug antibodies (ADAs) to trastuzumab or rHuPH20, a component of the SC Herceptin formulation. The percentage of participants who were trastuzumab ADA-positive and the percentage of participants who were rHuPH20 ADA-positive were each reported.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, IV Herceptin was given, and during Cycles 5 to 8, SC Herceptin was administered via SID. In the continuation period, participants received IV Herceptin for up to 10 remaining cycles. Administration was performed by HCP. Those with at least 2 treatment cycles remaining of the 18-cycle treatment course after the crossover period were offered the opportunity to self-administer SC Herceptin via SID under the direction of a trained HCP. The IV dose was a loading dose of 8 mg/kg in Cycle 1 for de novo participants who started Herceptin treatment in the study, and a dose of 6 mg/kg for all subsequent cycles where IV Herceptin was given and for non-de novo participants. The SC dose was 600 mg for all cycles where SC Herceptin was given.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, SC Herceptin was administered via handheld syringe using the vial formulation, and during Cycles 5 to 8, IV Herceptin was given. In the continuation period, participants received SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration was performed by HCP throughout the study. The SC dose was 600 mg for all cycles where SC Herceptin was given, and the IV dose was 6 mg/kg for all cycles where IV Herceptin was given.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, IV Herceptin was given, and during Cycles 5 to 8, SC Herceptin was administered via handheld syringe using the vial formulation. In the continuation period, participants received SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration was performed by HCP throughout the study. The IV dose was a loading dose of 8 mg/kg in Cycle 1 for de novo participants who started Herceptin treatment in the study, and a dose of 6 mg/kg for all subsequent cycles where IV Herceptin was given and for non-de novo participants. The SC dose was 600 mg for all cycles where SC Herceptin was given.

The estimated proportion of participants who preferred SC Herceptin and the corresponding exact binomial CI were determined.

The estimated proportion of participants who preferred SC Herceptin and the corresponding CI were determined using logistic regression with factors of previous Herceptin status and treatment.

The estimated proportion of participants who preferred SC Herceptin and the corresponding exact binomial CI were determined.

The estimated proportion of participants who preferred SC Herceptin and the corresponding CI were determined using logistic regression with factors of previous Herceptin status and treatment.

The estimated proportion of participants who preferred SC Herceptin and the corresponding exact binomial CI were determined.

The estimated proportion of participants who preferred SC Herceptin and the corresponding CI were determined using logistic regression with factors of previous Herceptin status and treatment.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, SC Herceptin was administered via handheld syringe using the vial formulation, and during Cycles 5 to 8, IV Herceptin was given. In the continuation period, participants received SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration was performed by HCP throughout the study. The SC dose was 600 mg for all cycles where SC Herceptin was given, and the IV dose was 6 mg/kg for all cycles where IV Herceptin was given.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, IV Herceptin was given, and during Cycles 5 to 8, SC Herceptin was administered via handheld syringe using the vial formulation. In the continuation period, participants received SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration was performed by HCP throughout the study. The IV dose was a loading dose of 8 mg/kg in Cycle 1 for de novo participants who started Herceptin treatment in the study, and a dose of 6 mg/kg for all subsequent cycles where IV Herceptin was given and for non-de novo participants. The SC dose was 600 mg for all cycles where SC Herceptin was given.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, SC Herceptin was administered via handheld syringe using the vial formulation, and during Cycles 5 to 8, IV Herceptin was given. In the continuation period, participants received SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration was performed by HCP throughout the study. The SC dose was 600 mg for all cycles where SC Herceptin was given, and the IV dose was 6 mg/kg for all cycles where IV Herceptin was given.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, IV Herceptin was given, and during Cycles 5 to 8, SC Herceptin was administered via handheld syringe using the vial formulation. In the continuation period, participants received SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration was performed by HCP throughout the study. The IV dose was a loading dose of 8 mg/kg in Cycle 1 for de novo participants who started Herceptin treatment in the study, and a dose of 6 mg/kg for all subsequent cycles where IV Herceptin was given and for non-de novo participants. The SC dose was 600 mg for all cycles where SC Herceptin was given.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, SC Herceptin was administered via handheld syringe using the vial formulation, and during Cycles 5 to 8, IV Herceptin was given. In the continuation period, participants received SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration was performed by HCP throughout the study. The SC dose was 600 mg for all cycles where SC Herceptin was given, and the IV dose was 6 mg/kg for all cycles where IV Herceptin was given.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, IV Herceptin was given, and during Cycles 5 to 8, SC Herceptin was administered via handheld syringe using the vial formulation. In the continuation period, participants received SC Herceptin via handheld syringe using the vial formulation for up to 10 remaining cycles. Administration was performed by HCP throughout the study. The IV dose was a loading dose of 8 mg/kg in Cycle 1 for de novo participants who started Herceptin treatment in the study, and a dose of 6 mg/kg for all subsequent cycles where IV Herceptin was given and for non-de novo participants. The SC dose was 600 mg for all cycles where SC Herceptin was given.

Participants received Herceptin on Day 1 of each 3-week cycle for 18 cycles. During Cycles 1 to 4 of the crossover period, IV Herceptin was given, and during Cycles 5 to 8, SC Herceptin was administered via SID. In the continuation period, participants received IV Herceptin for up to 10 remaining cycles. Administration was performed by HCP. Those with at least 2 treatment cycles remaining of the 18-cycle treatment course after the crossover period were offered the opportunity to self-administer SC Herceptin via SID under the direction of a trained HCP. The IV dose was a loading dose of 8 mg/kg in Cycle 1 for de novo participants who started Herceptin treatment in the study, and a dose of 6 mg/kg for all subsequent cycles where IV Herceptin was given and for non-de novo participants. The SC dose was 600 mg for all cycles where SC Herceptin was given.