Incidence and risk factors of lower respiratory tract illnesses during infancy in a Mediterranean birth cohort.

Aim: To investigate the incidence rate, viral respiratory agents and determinants of lower respiratory tract illnesses (LRTIs) in infants younger than 1 year.

Methods: A total of 487 infants were recruited at birth for the Asthma Multicenter Infant Cohort Study in Barcelona (Spain). Cases of LRTIs were ascertained through an active register including a home visit and viral test in nasal lavage specimens during the first year of life. Cotinine in cord blood, household aeroallergens, indoor NO(2) and maternal and neonatal IgE were measured. Other maternal and infants' characteristics were obtained from structured questionnaires.

Results: The incidence rate of at least one LRTI was 38.7 infants per 100 persons-years. The most frequently isolated viral agent was respiratory syncytial virus (44.7%). The risk of LRTIs was higher in infants with a maternal history of asthma and in those with siblings (OR = 2.4; 95% CI: 0.98-6.08 and OR = 1.8; 95% CI: 1.04-3.21, respectively). The risk of LRTIs was lower in infants who were breast fed for more than 12 weeks (OR = 0.26; 95% CI: 0.26-0.86) and in those from a low socioeconomic class (OR = 0.16; 95% CI: 0.06-0.42).

Conclusion: Viral LRTIs are frequent in infants younger than 1 year of age and there is an inter-relationship between maternal asthma, siblings, breast feeding and socioeconomic status.