e23114 Background: Little is known about the impact of the COVID-19 pandemic on post-treatment surveillance for patients diagnosed with breast cancer. Disruptions in recommended cancer surveillance screening to detect progression or recurrence could impact long-term outcomes. In this study, we compared use of recommended surveillance screening in pre- and post-COVID time periods for patients diagnosed with breast cancer within a large integrated healthcare system, Kaiser Permanente Southern California (KPSC). We also evaluated detection of breast cancer recurrence or progression. Methods: We identified patients in post-treatment surveillance for early-stage breast cancer in two time periods, pre-COVID and post-COVID. Pre-COVID included patients diagnosed 01/01/2015-12/31/2016 with 2 years of follow-up time through 12/31/2019; post-COVID included patients diagnosed 06/01/2018-05/31/2020 with 2 years of follow-up time through 05/31/2023. Eligible patients were diagnosed with stage 0-II breast cancer and received curative surgery. We excluded patients with multiple cancers, and those who disenrolled from the health plan, died, or were referred to hospice within the 2-year surveillance period. Based on NCCN guidelines, we identified the proportion of patients who underwent annual breast imaging during each year of the surveillance period. A log Poisson model examined the association between imaging and COVID period; covariates included patient age, race/ethnicity, cancer stage, treatment, and comorbidity score. We identified cancer recurrence or progression using diagnostic, treatment, and imaging codes. Results: We identified 3,369 pre-COVID patients and 3,925 post-COVID patients; groups were similar (Table 1). A significantly smaller proportion of post-COVID patients received surveillance screening (first surveillance year: 85% in the pre-COVID group vs. 77% post-COVID, p = 0.001; second year: 83% vs. 80%, p = 0.01). Patients were 4% less likely to receive imaging post-COVID compared to pre-COVID (adjusted OR 0.96, 95% CI: 0.94-0.98). Similar percentages of patients had breast cancer recurrence or progression in the pre-COVID (2.7%) and post-COVID (2.5%) periods (p-value = 0.44). Conclusions: Fewer post-treatment breast cancer patients received recommended annual breast cancer screening for surveillance in the post-COVID period compared to pre-COVID, but breast cancer recurrence or progression was similar between groups. Given the less frequent surveillance in the post-COVID period, cancer recurrence and progression should be evaluated over 5- and 10-year periods as data become available. [Table: see text]