ERBB2 and breast cancer: Based on the deterministic analysis, from the societal perspective, adjuvant trastuzumab therapy combined with chemotherapy compared to chemotherapy alone was estimated to incur an additional lifetime cost of 4,216,100 million VND (corresponding to 185,363,845 US$, which equals 0.053% of total GDP in 2020 in Vietnam) with an additional lifetime health gain of 15,618 LYs or 8,113 QALYs for the whole hypothetical population of 5,052 early-stage breast cancer women with HER2+ in Vietnam (Table 2).