OBJECTIVE. The goal was to investigate the epidemiological features of incident bronchiolitis by using a population-based infant cohort.

METHODS. Outpatient and inpatient health records were used to identify incident bronchiolitis cases among 93 058 singleton infants born in the Georgia Air Basin between 1999 and 2002. Additional health-related databases were linked to provide data on sociodemographic variables, maternal characteristics, and birth outcome measures.

RESULTS. From 1999 to 2002, bronchiolitis accounted for 12 474 incident health care encounters (inpatient or outpatient contacts) during the first year of life (134.2 cases per 1000 person-years). A total of 1588 hospitalized bronchiolitis cases were identified (17.1 cases per 1000 person-years). Adjusted Cox proportional-hazard analyses for both case definitions indicated an increased risk of incident bronchiolitis in the first year of life (follow-up period: 2–12 months) for boys, infants of First Nations status, infants with older siblings, and infants living in neighborhoods with smaller proportions of maternal postsecondary education. The risk also was elevated for infants born to young mothers (<20 years of age) or mothers who did not initiate breastfeeding in the hospital. Infants with low (1500–2400 g) or very low (<1500 g) birth weight and those with congenital anomalies also had increased risk. Maternal smoking during pregnancy increased the risk of hospitalized bronchiolitis.

CONCLUSIONS. This population-based study of the epidemiological features of bronchiolitis provides evidence for intervening with high-risk infants and their families. Clinical and public health interventions are recommended for the modifiable risk factors of maternal breastfeeding and smoking and for modification of vulnerable environments where possible (eg, limiting exposure to other young children), during high-risk periods such as the first few months of life or the winter season.

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