Pharmacoepidemiology
Postpartum COVID-19 vaccination and risk of lactational mastitis Yihe G Daida* Yihe Daida Daida Daida Daida Daida Daida Daida Daida Daida Daida Daida Kaiser Permanente Hawaii
Purpose: Mastitis could occur as a vaccination-related adverse effect, if vaccination were associated with reduced breastfeeding frequency, leading to subsequent inflammation. We aimed to assess the association between maternal postpartum COVID-19 vaccination and risk of lactational mastitis.
Method: We conducted a retrospective study using electronic health record (EHR) data from four integrated health systems. Eligible patients had a live birth between December 2020–September 2022, were linked to an infant with ≥1 well visit before 60 days of life and had documentation of ever breastfeeding in the EHR. State immunization information system supplemented EHR data on COVID-19 vaccination. The primary outcome was an International Classification of Diseases, Tenth Revision (ICD-10) Clinical Modification mastitis diagnosis (N61.0 or O91.2x) in the 180 days postpartum; the secondary outcome was mastitis plus antibiotic dispensing within 3 days. Cox proportional hazards models with time-dependent exposure estimated adjusted hazards ratios (aHR) and 95% CI, accounting for site, maternal age, insurance, race and ethnicity, infant gestational age at birth, smoking status, neighborhood-level socioeconomic characteristics (education, poverty level, medium household income) and year and month of delivery with stabilized inverse probability of treatment weights. Person-time was classified as unexposed until vaccination and as exposed thereafter. Patients were censored at first mastitis diagnosis or 180 days postpartum, whichever occurred first.
Results: Among 18,446 individuals; 36.2% were vaccinated in the 180 days postpartum. Overall, 5.5% were diagnosed with mastitis. There was no significant association between vaccination and lactational mastitis (aHR 1.10, 95% CI: 0.83-1.46) or mastitis diagnosis plus antibiotic dispensing (aHR 1.17, 95% CI: 0.87-1.59).
Conclusion: COVID-19 vaccination was not associated with a higher lactational mastitis risk in the 6 months after delivery.