Baseline characteristics of the study population
| Total participants n (% male) | 167 (57) |
| Mean±sd gestational age weeks | 39.3±2.0 |
| Mean±sd birth weight kg | 3.3±0.55 |
| Caesarean section | 25 (15) |
| Season of birth | |
| Spring | 62 (37) |
| Summer | 44 (26) |
| Autumn | 32 (19) |
| Winter | 29 (17) |
| Mean±sd (range) age at ARI weeks | 29.9±12.5 (2–52) |
| Season at first ARI | |
| Spring | 40 (24) |
| Summer | 14 (8) |
| Autumn | 38 (23) |
| Winter | 75 (45) |
| Increased atopy risk | 109 (65) |
| Childcare attendance | 32 (19) |
| Breastfeeding status at time of ARI# | 99 (60) |
| Environmental tobacco smoke exposure | 24 (14) |
| Other siblings (living in the same household) | 106 (63) |
| One sibling | 75 (45) |
| ≥2 siblings | 31 (19) |
Data are presented as n (%), unless otherwise stated. ARI: acute respiratory tract infection. #: infants being breastfed at the time of ARI.