Wu P, Dupont WD, Griffin MR, et al. Am J Respir Crit Care Med. 2008;178(11):1123–1129
PURPOSE OF THE STUDY. In the first year of life, ∼20% of children have ≥1 episode of respiratory illness with wheezing. Other studies have shown that certain respiratory viruses confer an increased risk of developing later childhood asthma. Whether these common respiratory viruses cause asthma or are a marker of individuals predisposed to developing asthma is unknown. The timing of birth in relation to the winter virus peak and whether this alters the risk of developing early childhood asthma are investigated in this study.
STUDY POPULATION. A population-based, birth cohort study of 95 310 children who were born between 1995 and 2000 and followed through 2005, who were continuously enrolled in the Tennessee Medicaid program from birth through early childhood, representing 25% of the annual births in Tennessee, was performed.
METHODS. The criteria for defining asthma variables and classification represent a minor flaw, because they were not defined a priori and were based on adult data. However, the authors used well-designed methods to make certain that the main outcome variables were defined in the best way the data allowed. Infant birth in relation to the winter virus peak was defined for each infant as the infant's age in days from birth to the first winter virus peak. The annual winter virus peak was defined as the first day of the week with the highest number of bronchiolitis hospitalizations for that winter season.
RESULTS. During the 5 winter virus seasons, the risk of developing asthma tracked with the timing of infant birth in relation to the winter virus peak among the 95 310 children studied from birth through early childhood. Infant birth ∼4 months before the winter virus peak carried the highest risk, with a 29% increase in the odds of developing asthma, compared with birth 12 months before the peak (odds ratio: 1.29). Infant age at the winter virus peak was comparable to or greater than other known risk factors for asthma, such as maternal smoking or maternal asthma. Over the 5 study seasons, the increase in the incidence of bronchiolitis during infancy paralleled the subsequent increase in the cumulative incidence of current high-risk asthma defined between 4 and 5.5 years of age and offset by 5 years from the birth season in the same cohort of children.
CONCLUSIONS. This study demonstrates that the timing of birth in relation to the winter virus season confers a differential and definable risk of developing early asthma, establishing winter virus seasonality as a causal factor in asthma development. The authors have demonstrated that increasing rates of infant bronchiolitis in the past 10 years parallel the 5-year–offset increases in asthma at 5 years of age among these children.
REVIEWER COMMENTS. Timing of birth in relation to the winter virus peak independently predicts asthma development, with the highest risk estimated to be birth at 121 days before the winter virus peak of any given year. This age confers a 29% increased risk of developing childhood asthma. These findings, taken with findings from the Childhood Origins of Asthma study (reviewed above), are exciting and will fuel the debate about interventions for avoiding viral infections in infancy (are they a creditable target?). There is controversy in this area, because children who attend day care centers and who are more exposed to recurrent respiratory infections have been shown to have less asthma later in life. Possibly some subgroups at greater risk of developing asthma, such as infants who are young at the beginning of the virus season and those with an atopic background, may benefit most from preventive and therapeutic interventions.
- Copyright © 2009 by the American Academy of Pediatrics