Allergic outcomes
Data on parent-reported doctor diagnosis of asthma and allergic rhinitis (AR) were collected at ages 0.5, 1, 1.5, 2, 4, 6, 10 and 15. If the time between follow-ups was longer than one year (e.g., 10 to 15 years), the presence of the diagnosis was asked separately for each year since the previous follow-up (e.g., 10, 11, 12, 13, 14 and 15). In line with our previous analyses, 17 only information collected from age 3 onwards was used due to difficulty of accurate diagnosis of these outcomes at younger ages. Asthma at ages Y = 3, 4, … 15 was defined as a positive response to “Was your child diagnosed with asthma by a doctor at the age of Y years?” AR at ages Y = 3, 4, … 15 was defined as a positive response to “Was your child diagnosed with hay fever/allergic rhinitis by a doctor at the age of Y years?”
Specific immunoglobulin E (IgE) against common allergens was assessed at ages 2, 6, 10 and 15 using the standardized CAP-RAST FEIA method (ThermoFischer, Freiburg, Germany). At 2 years, allergic sensitization was tested to tree and grass pollen (RX1), mold, cat and house dust mites. At 6, 10 and 15 years, allergic sensitization was tested by using inhalant mix SX1, which consists of house dust mite, cat, dog, mold, birch, rye, mugwort and timothy grass allergens. Allergic sensitization to aeroallergens was defined as IgE > 0.35 kU/L for at least one of the tested inhalant allergens at 2 years and for the SX1 mix at the other timepoints. Allergic sensitization to food allergens was defined as IgE > 0.35 kU/L for allergens in the FX5 screening test (milk, peanut, eggs, soya, cod and wheat flour) at all timepoints.