AIM: To analyse the impact of the COVID-19 pandemic on the sustainability of a breastfeeding (BF) clinical practice guideline (CPG) for women without COVID-19, throughout the 5 waves of the pandemic. DESING: A mixed-methods design was utilised. METHODS: For the quantitative approach, an interrupted time series design was utilised, as well as the analysis of CPG sustainability reports as a qualitative approach. The study setting was in a health area in the Spanish health system from April 2019, until October 2021. The sample was composed of 2239 mother-infant dyads. RESULTS: The exclusive-BF rate at hospital discharge obtained values between 90% and 94.8%, without statistically significant changes. A significant increase in the risk of not starting BF in the first feeding was observed (adjusted odds ratio = 9.36
95% CI: 1.04-84.13), between the pre-pandemic period and the first wave. Skin-to-skin contact (SSC) decreased in the first wave to 82.20%, and the oscillations observed throughout the pandemic were not statistically significant. In general, the qualitative indicators were maintained. A decrease was observed in the spaces used for postpartum care due to the re-assigning to the intensive care unit. Also, the acquisition of materials and equipment decreased. CONCLUSIONS: The measures implemented for the sustainability of the BF CPG during the 5 waves of the pandemic were positive. The programmes of implementation of BF guidelines were shown to resist the COVID-19 pandemic. IMPACT: Our findings contribute to the understanding and evolution of the main indicators of the sustainability of a BF CPG on COVID-19 context, providing details on the magnitude of the effect and the process of change. REPORTING METHOD: The Preferred Reporting Items for observational studies (STROBE) checklist was followed. PATIENT OR PUBLIC CONTRIBUTION: No Patient or Public Contribution.