On the association between date of birth and pollen sensitization: is age an effect modifier?

An association between date of birth and development of allergy has been proposed by prior research. Yet, the presence of a dose-response relationship or any potential effect modification for this association has not been widely studied. The aims of our study were to investigate whether an association between birth during pollen season and symptomatic sensitization to pollens exists, whether this association is stronger for patients with high rather than low allergic reactivity to pollens, and whether this association is modified by the age of the patients. Among 3318 asthmatic and/or rhinitic outpatients, we selected 805 patients sensitized exclusively to pollens (78 with low reactivity [LR] and 727 with high reactivity [HR]) and 629 patients with negative skin-prick tests (SPT) (control group). The association between being born during pollen season (February-July) and each of the pollen reactivity levels was assessed by estimating the odds ratios (OR). HR pollinosis patients were more likely than SPT negative patients of being born in February-July (OR 1.38, 95% Confidence Intervals (CI) 1.11-1.71). The likelihood of having been born in pollen season significantly increased across the levels of reactivity to pollens (HR > LR > SPT negative). These findings were valid only among patients with an early onset of symptoms. Although the OR for being born in pollen season was 1.91 (95% CI 1.32-2.77) for HR pollinosis patients with onset of symptoms < or = 15 years, it was 1.13 (95% CI 0.87-1.48) for those with later onset of symptoms (test of homogeneity: p = 0.026). Our results suggest that the exposure to allergenic pollens in the first months of life increases the risk of developing clinically relevant sensitization to them, particularly in the first 15 years of life.