BACKGROUND: Randomised controlled trials (RCTs) reported adjuvant trastuzumab-based treatment improved overall survival (OS) among patients with HER2-positive early invasive breast cancer (EIBC). Few RCTs included older patients or those with comorbidity/frailty. This study aimed to determine whether the effect of adjuvant trastuzumab-based treatment on survival outcomes varies by patient age and fitness, using national data from routine care. METHODS: Women (50+ years) newly-diagnosed with HER2-positive EIBC between 2014 and 2019 were identified from England Cancer Registry data. Registration records were linked to Systemic Anti-Cancer Therapy data for treatment details and ONS death register for mortality details. A propensity score analysis employing the inverse probability of treatment weighting method was used to balance the patient variables across treatment groups. Cox models were used to evaluate whether the effect of treatment on OS was associated with patient age and fitness; competing risks regression models were used for breast cancer-specific survival (BCSS). RESULTS: 5238 women initiated adjuvant trastuzumab-based treatment. Median follow-up was 56.7 months. Comparison with 3421 women who did not receive adjuvant trastuzumab highlighted differences at diagnosis in relation to age, fitness, grade, nodal involvement, surgery type and use of radiotherapy. Weighted survival analysis found trastuzumab was associated with improved OS (hazard ratio HR 0.56, 95 %CI: 0.45-0.70) and improved BCSS (subHR 0.62, 95 %CI: 0.47-0.82). We found no evidence of a difference in effect by age or patient fitness for either outcome. CONCLUSION: In this national dataset, adjuvant trastuzumab was associated with improvements in survival, with an OS effect size similar to RCT evidence. The effect size was not found to vary by patient age or fitness. Chronological age and fitness alone should not be barriers to receipt of effective adjuvant targeted treatment.