AbstractBackgroundVitamin D has immune‐modulating effects. We determined whether vitamin D supplementation during pregnancy and infancy prevents aeroallergen sensitization and primary care respiratory illness presentations.MethodsA randomized, double‐blind, placebo‐controlled parallel‐group trial. We assigned pregnant women, from 27‐week gestation to birth, and then their infants, from birth to 6 months, to placebo or one of two dosages of daily oral vitamin D. Woman/infant pairs were randomized to: placebo/placebo, 1000 IU/400 IU or 2000 IU/800 IU. When the children were 18 months old, we measured serum‐specific IgE antibodies and identified acute primary care visits described by the doctor to be due to a cold, otitis media, an upper respiratory infection, croup, asthma, bronchitis, bronchiolitis, a wheezy lower respiratory infection or fever and cough.ResultsSpecific IgE was measured on 185 of 260 (71%) enrolled children. The proportion of children sensitized differed by study group for four mite antigens: Dermatophagoides farinae (Der‐f1, Der‐f2) and Dermatophagoides pteronyssinus (Der‐p1, Der‐p2). With results presented for placebo, lower dose, and higher dose vitamin D, respectively (all P < 0.05): Der‐f1 (18%, 10%, 2%), Der‐f2 (14%, 3%, 2%), Der‐p1 (19%, 14%, 3%) and Der‐p2 (12%, 2%, 3%). There were study group differences in the proportion of children with primary care visits described by the doctor as being for asthma (11%, 0%, 4%, P = 0.002), but not for the other respiratory diagnoses.ConclusionsVitamin D supplementation during pregnancy and infancy reduces the proportion of children sensitized to mites at age 18 months. Preliminary data indicate a possible effect on primary care visits where asthma is diagnosed.