Disadvantage in early-life and persistent asthma in adolescents: a UK cohort study

Abstract

Objective: To determine how early-life risk factors explain socioeconomic inequalities in persistent asthma in adolescence.

Methods: We did a causal mediation analysis using data from 7487 children and young people in the UK Millennium Cohort Study. Persistent asthma was defined as having a diagnosis reported at any two or more time points at 7, 11 or 14 years. The main exposure was maternal education, a measure of early-life socioeconomic circumstances (SECs), used to calculate the relative index of inequality. We assessed how blocks of perinatal (maternal health behaviours, infant characteristics and duration of breastfeeding, measured at 9 months) and environmental risk factors (family housing conditions; potential exposure to infections through childcare type and sibling number, and neighbourhood characteristics, measured at 3 years) mediated the total effect of childhood SECs on persistent asthma risk, calculating the proportion mediated and natural indirect effect (NIE) via blocks of mediators.

Results: At age 14 the overall prevalence of persistent asthma was 15%. Children of mothers with lower educational qualifications were more likely to have persistent asthma, with a clear social gradient (degree plus: 12.8% vs no qualifications: 20.3%). The NIE gives the effect of SECs acting only via the mediators and shows a 31% increased odds of persistent asthma when SECs are fixed at the highest level, and mediators at the level which would naturally occur at the lowest SECs versus highest SECs (NIE OR 1.31, 95% CI 1.04 to 1.65). Overall, 58.9% (95% CI 52.9 to 63.7) of the total effect (OR 1.70, 95% CI 1.20 to 2.40) of SECs on risk of persistent asthma in adolescence was mediated by perinatal and environmental characteristics.

Conclusions: Perinatal characteristics and the home environment in early life are more important in explaining socioeconomic inequalities in persistent asthma in British adolescents than more distal environmental exposures outside the home.

Keywords: asthma; asthma epidemiology.

Figure 1

Logic model of the pathways from SECs at birth (maternal education captured at 9 months) to mid-childhood/adolescent asthma (7, 11 and 14 years), with direct pathway shown in bold, indirect pathways via mediators in dashed lines and baseline confounding pathways in dotted lines. Mediating blocks capture: (1) Perinatal characteristics at 9 months (maternal pre-pregnancy body mass index; breastfeeding duration; birth weight; gestation; maternal smoking; alcohol use during pregnancy; seen a health professional for wheeze?), (2) home environment at 3 years (damp/condensation; tenure; furry pets in household; environmental tobacco smoke; access to garden) (3) exposure to infection at 3 years (sibling number in household and informal vs formal childcare) (4) neighbourhood characteristics at 3 years (volume of traffic; housing quality; rubbish on pavement; good area to bring up children?). This logic model corresponds to data available in Millennium Cohort Study only and is not a complete directed acyclic graph. SECs, socioeconomic circumstances.

Figure 2

Flow diagram of cohort construction a productive cohort members have some data from one of six collection questionnaires at every data collection wave.

Figure 3

Mediation analysis with a counterfactual approach by cumulative blocks of mediators (perinatal characteristics, housing conditions, potential exposure to infection and neighbourhood) characteristics in the association between socioeconomic circumstance and persistent asthma.