Contagion Models for the Transmission of Drug Abuse Among Propinquity-of-rearing Defined Acquaintances: A Swedish National Study

Background: Can we validate a contagion model for drug abuse (DA) in Propinquity-of-Rearing Defined Acquaintances (PRDAs)?

Methods: PRDAs were defined as pairs of same-age males born 1975-1990 who grew up within 2 km of each other, one of whom (PRDA1) being first registered for DA in national registries. Using adult residential location, we predicted, using regression splines, proximity-dependent risk for DA first registration in a second PRDA (PRDA2) within 3 years of PRDA1's registration.

Results: In 181,743 PRDA pairs, the best-fit model, controlling for age and PRDA2 community risk, included 2 slopes of proximity-risk relationships in childhood and three in adulthood. Risk for DA in PRDA2 was strongly predicted by childhood proximity to PRDA1: 0 to 0.5 km - Hazard ratio (HR) per kilometer 0.52 and 0.6-2 km 0.78. HRs for PRDA2 as a function of adult proximity to PRDA1 were: 0-1 km 0.887, 1-75 km 0.996 and >75 km 0.9997. Proximity-dependent PRDA2 risk was moderated by age, familial risk and educational attainment, attenuated by increasing PRDA1-PRDA2 age differences and stronger for older to younger versus younger to older pairs.

Conclusions: Transmission of DA risk between acquaintances growing up together was attenuated by increasing distance in adulthood. Strength of the acquaintance, indexed by childhood propinquity and age difference, modified transmission strength. The impact of adult proximity on transmission was reduced in acquaintances with higher resistance to DA due to older age, higher educational attainment or lower familial risk. Our results support the validity of DA contagion models.