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Evaluation Video

Oral Presentation

Session: The Treatment and Prevention of Pregnancy-related Infections

Postpartum COVID-19 vaccination and risk of lactational mastitis

Monday, August 26, 2024
2:30 PM – 2:45 PM CEST
Location: Room XV
Background: There are limited data on lactation-related outcomes following postpartum COVID-19 vaccination. It is possible that COVID-19 vaccine side effects (e.g., fatigue, malaise) during lactation could decrease the frequency of human milk feeding and contribute to lactation difficulties, subsequently increasing the risk of lactational mastitis.

Objectives: To assess the association between postpartum COVID-19 vaccination and risk of lactational mastitis.

Methods: We conducted a retrospective study using electronic health record (EHR) data from a large integrated health care delivery system based in Minnesota, USA. We included patients who had a live birth during December 2020–September 2022, were able to be linked to their infant(s) who had ≥1 well-visit before 60 days of life, and had EHR documentation of ever breastfeeding in infant or maternal records. COVID-19 vaccination was identified from EHR-based vaccine registries supplemented with data from the Minnesota state immunization information system. The outcome was defined as an inpatient or outpatient International Classification of Diseases, Tenth Revision Clinical Modification (ICD-10)-CM diagnosis code for mastitis (N61.0 or O91.2x) in the 180 days after delivery. A secondary outcome was a mastitis diagnosis code with an antibiotic dispensing within 3 days of code assignment. We used Cox proportional hazards models with a time-dependent exposure to calculate hazard ratios (HR) and 95% confidence intervals (CI) adjusted for maternal age, public insurance, race and ethnicity, and gestational age at delivery. Person-time, in days since delivery, was classified as unexposed until receipt of COVID-19 vaccine and was classified as exposed thereafter. Patients were censored on the day of their first mastitis diagnosis or at 180 days after delivery, whichever occurred first.

Results: There were 6,985 patients in the study population, and 35% received COVID-19 vaccine in the 180 days after delivery. Overall, the risk of lactational mastitis was 5%. The adjusted HR between COVID-19 vaccination and lactational mastitis was 1.19 (95% CI: 0.88, 1.61) and was 1.22 (95% CI: 0.90, 1.67) when defining the outcome as a diagnosis for mastitis plus an antibiotic dispensing. Among patients with a mastitis diagnosis following COVID-19 vaccination, the median number of days between vaccination and first mastitis diagnosis was 35 (interquartile range: 65).

Conclusions: There was not a statistically significant increased risk of lactational mastitis following COVID-19 vaccination in the 6 months after delivery. Analyses will be updated to include data from additional health systems.