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Respiratory infections in preterm infants and subsequent asthma: a cohort study
  1. Scott Montgomery1,2,3,
  2. Shahram Bahmanyar2,4,
  3. Ole Brus1,
  4. Oula Hussein1,
  5. Paraskevi Kosma5,6,
  6. Charlotte Palme-Kilander5
  1. 1Clinical Epidemiology and Biostatistics, Örebro University Hospital & Örebro University, Örebro, Sweden
  2. 2The Clinical Epidemiology Unit, Karolinska Institutet, Stockholm, Sweden
  3. 3Department of Epidemiology and Public Health, University College London, UK
  4. 4The Centre for Pharmacoepidemiology, Karolinska Institutet, Stockholm, Sweden
  5. 5Department of Woman and Child Health, Karolinska Institutet, Stockholm, Sweden
  6. 6Division of Neonatology, Karolinska Hospital, Stockholm, Sweden
  1. Correspondence to Professor Scott Montgomery, Clinical Epidemiology and Biostatistics, S-huset, Örebro University Hospital, Örebro 701 85, Sweden; Scott.montgomery{at}


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 5 years.

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

Participants National registers identified 42 334 children admitted to hospital for respiratory infection in their first year after birth during 1981–1995, individually matched with 211 594 children not admitted to hospital for infection during their first year.

Primary outcome Asthma diagnoses and prescribed asthma treatments after the age of 5 years 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 1 year of age with asthma after age 5 years, 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 10 years, but differences in risk by gestational age were less pronounced for asthma after age 16 years.

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

  • gestational age
  • bronchiolitis
  • cohort study

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