BACKGROUND: Previous studies of early day care attendance and asthma development are inconsistent, which may be explained by inadequate control of confounding and effect modification. We examined the effect of early day care on the risk of asthma taking into account the underlying susceptibility to asthma.
METHODS: The study included 55,404 children participating in the Norwegian Mother, Father and Child Cohort Study. Asthma at age 7 was defined by dispensed asthma medications in the Norwegian Prescription Database. We defined a disease risk score (DRS) to account for an underlying susceptibility to asthma including a range of hereditary and non-hereditary predictors of asthma. We assessed confounding and modifying effects of DRS on the association between day care and asthma.
RESULTS: Day care before 18 months was associated with a lower risk of asthma by age 7 (adjusted risk ratio (RR) 0.85; 95% CI 0.78-0.92) when compared to home care. DRS modified the estimated effect of day care on asthma risk. Among the 80% of children with DRS between 0.03 and 0.16, day care was associated with a reduced asthma risk (RRs between 0.79-0.87), whereas among 0.5% of children with a high DRS (above 0.28), estimated effect of day care on asthma increased gradually (RR for the highest DRS 2.2; 1.0-4.9).
CONCLUSIONS: In our study, among most children, early day care was associated with reduced asthma risk at 7 years, and increased risk in a small group of children with very high underlying susceptibility to asthma.