To Örebro University

Objectives: To investigate whether gestational age modifies the association of airway infections that result in hospital admission during the first year after birth, with subsequent asthma risk after age 5years.

Setting: Hospital inpatients and a general population comparison group in Sweden followed for subsequent diagnoses in primary and secondary care.

Participants: National registers identified 42334 children admitted to hospital for respiratory infection in their first year after birth during 1981-1995, individually matched with 211594 children not admitted to hospital for infection during their first year.

Primary outcome Asthma diagnoses and prescribed asthma treatments after the age of 5years identified through registers.

Results: Cox regression was used to identify a HR (and 95% CI) of 1.51 (1.47 to 1.51) for the association of respiratory infection before 1year of age with asthma after age 5years, after adjustment for sex, gestational age, chronic lung disease, maternal asthma and maternal smoking. When stratified by gestational age (and with additional adjustment for birth weight), there is statistically significant effect modification by gestational age, with the highest magnitude asthma risk among those born with a gestational age of less than 28 weeks, producing an adjusted HR of 2.22 (1.59 to 3.09). This higher magnitude asthma risk persisted until after age 10years, but differences in risk by gestational age were less pronounced for asthma after age 16years.

Conclusions: Extremely preterm infants are most likely to have chronic respiratory sequelae following respiratory infections in early life.