Parent-driven transmission, off-label SMZ-TMP, and severe pertussis prediction in infants under 2 months: a real-world observational cohort of 527 cases.

BACKGROUND: Despite high DTaP coverage, pertussis resurgence persists. Infants under 2 months of age face severe outcomes due to vaccine ineligibility and rising macrolide-resistant B. pertussis (MRBP), yet treatment options remain limited. OBJECTIVE: To analyze epidemiology, clinical profiles, and antimicrobial management of hospitalized pertussis infants under 2 months of age, identifying risk factors for severity and evaluating real-world SMZ-TMP use. METHODS: Retrospective study of PCR/culture-confirmed pertussis infants hospitalized at Shenzhen Children's Hospital (2015-2024). Data included transmission sources, clinical severity, and antimicrobial regimens. Logistic regression analysis was performed to analyze the risk factors for severe pertussis, and a P-value < 0.05 was considered statistically significant. The predictive value for severe pertussis was evaluated using the receiver operating characteristic (ROC) curve. RESULTS: Among 321 infants with a history of exposure, parents constituted the primary infection source (46.1%, 148/321). 25 infants with Bordetella pertussis infection (aged 24-59 days) received off-label SMZ-TMP without bilirubin encephalopathy or renal dysfunction. Leukocytosis, and an elevated neutrophil-lymphocyte ratio were predictors of severe disease. Exposure history, apnea, decreased oxygen saturation, tachypnea, tachycardia, elevated white blood cell count, and an increased neutrophil-to-lymphocyte ratio were identified as significant risk factors for severe pertussis. CONCLUSION: In infants under 2 months of age with pertussis, parents are the primary source of infection. In infants under 2 months of age with macrolide-resistant pertussis, treatment with SMZ-TMP was effective and no severe adverse reactions were observed.