Rationale: We have previously reported an increase in salivary cortisol in a cohort of 4-year-old children transitioning to primary school. We hypothesised that increased cortisol in response to this acute naturalistic stress in early development may be immunostimulatory and associated with positive health outcomes.
Objectives: We tested this hypothesis by measuring upper respiratory tract infection (URI) across the first 6 months of school, in relation to salivary cortisol at the end of the second week following school transition.
Methods: Seventy children supplied morning and evening saliva samples for cortisol assay. Children were psychologically assessed for temperament and behavioural adaptation. Symptoms of URI were recorded in diary form, and variables relating to URI occurrence, duration and severity were assessed.
Results: Children with higher evening cortisol at school transition experienced significantly fewer episodes of URI over the following 6 months. Diurnal cortisol change was negatively correlated with number of illnesses across the 6 months, indicating an association between a greater decline in cortisol across the day and a greater number of colds. URI severity was associated with the greatest resistance to URI infection in children who were less socially isolated and who had a smaller diurnal change in cortisol across the day.
Conclusions: Our results showing that higher cortisol is associated with lower URI may be explained by proposing that increased cortisol in response to the naturalistic stress of school transition may prime the immune system to develop resistance to URI at this critical stage of a child’s development.