Epidemiology of illicit drug use and policy evaluation: the heroin epidemics example

Researchers have recognized, for over half a century, the similarities of the spread of drug use and that of infectious diseases; the equivalent of "contagion" being the "transmission of knowledge" of this ‘innovative’ social practice from an individual to another.

The epidemiological approach, deriving from such assumption, makes it possible to model available administratve data, regarding “visible” consequences of drug use, and carry out the impact assessments of drug policies and short and medium term forecasts of visible and hidden measures to be estimated such as prevalence and specific and incidence.

A crucial technical point concerns models with "closed populations" or with "open populations”. The former consider fixed populations, without entry of new individuals (births, immigrations etc.), and are simpler but allow for only short-term forecasts; while the others, which consider also at least birth inputs in the population, are more complex, but also allow forecasts of medium and long-term (at least qualitative incidence and prevalence behaviours), and preventive impact analysis of policy interventions (prevention, repression, harm reduction etc.), providing information to decision makers.

The epidemics of infectious diseases and drug consumption also share an important qualitative characteristic, well highlighted by dynamic models with open populations: the presence of "waves" after an initial eruption, followed by endemic trends, and then new epidemic waves in the absence of effective control interventions (prevention and vaccinations for diseases; prevention, repression, treatment and harm reduction for drug use).

The first epidemiological models for the spread of illegal substances are dedicated in the ‘90s to the first epidemic wave of heroin use around the world (USA, West Europe, specific countries..). Most of them are models with closed populations and useful to estimate some unknown parameter (incidence, prevalence..) and to make short term forecasts to evaluate, for example, the need for specific therapies in the next 5 years. Only a few models proposed for the spread of heroin are open populations models and allow to assess in advance the effectiveness of possible control interventions with medium and long term forecsts, at least qualitatively.

The recent data about the visible consequences of heroin use (overdoses, emergency interventions, therapy input etc.) and Eurostat's recent European estimates on the strongly growing heroin market (in Italy a 42% growth between 2011 and 2015) show that a second epidemic wave of heroin use is currently under way in the USA and in European countries. Such a wave was forecasted under different simulated scenarios of heroin epidemic models with open populations (Rossi "MoverStayer" 2001, 2004), later about a generation from the first, in absence of effective prevention interventions and of effective therapy interventions (especially harm reduction) in the years following the insurgence of the first epidemic wave. This is the effect shown by empirical data in USA and Europe. But in Switzerland, only an endemic trend is definitely observed. The substantial difference in drug policy is the so-called "controlled heroin therapy" in progress in Switzerland for 25 years and since 1997 as a usual therapeutic intervention. The main conclusionis is that everywhere it would be important to introduce soon such therapy regularly. Politicians must have antiprohibitionist approach to therapy.