Background: A recent clinical trial (Impassion131) showed that addition of atezolizumab to paclitaxel does not provide survival benefit for first-line (1L) treatment of metastatic triple-negative breast cancer (mTNBC) patients. The study was in contradiction to a previous study (Impassion130) that did show benefit using atezolizumab plus nab-paclitaxel. Using real-world data, we evaluated the impact on overall survival and post-treatment hospitalizations of 1L Atezolizumab + Taxane (Nab-Paclitaxel) (A+T) vs. Taxane agents alone (Paclitaxel or Nab-Paclitaxel) (T) in individuals with mTNBC. Methods: We performed a retrospective cohort study of 1L mTNBC patients between 1/2016 and 6/2021, utilizing administrative claims data from the Anthem Cancer Care Quality Program. Real-world overall survival (rwOS) was defined as time from diagnosis to death. Cox model with inverse probability of treatment weighting was used to adjust for age, ECOG, Socioeconomic deprivation index (SDI), comorbidity, and liver and bone metastatic sites. Median rwOS was estimated using the weighted Kaplan-Meier method. Adjusted hazard ratios (aHR) were estimated using weighted Cox proportional hazards models. We do not adjust for PD-L1 levels shown to have association with improved OS regardless of treatment, therefore, unlikely to confound our results. Results: The A+T combination therapy became a preferred 1L type of treatment for mTNBC patients since its introduction in 2019. We identified 155 (55%) individuals with 1L mTNBC who received A+T and compared this cohort to 128 (45%) individuals who received T alone. The A+T group were younger and had lower proportion of bone metastatic sites (Table). Percentage of patients with previous chemotherapy in the last year was similar between the groups (p = 0.9). Median rwOS was 13.6 and 17.5 months, in A+T and T group, respectively (p = 0.2). In adjusted survival analysis, the addition of atezolizumab showed insignificant increase in the risk of mortality (aHR: 1.09 (0.74-1.61), p = 0.66). Our post-treatment analysis did not identify statistically significant differences between the groups, in terms of post-treatment hospitalizations, adverse events classes, or length of stay of hospitalized patients. Conclusions: Our retrospective cohort study demonstrated that the addition of atezolizumab to 1L treatment, was not associated with survival benefit or reduce post-treatment adverse events in mTNBC patients.