MODELING THE ANNUAL RISK OF TUBERCULOSIS INFECTION

Background. Tuberculosis has been declining in developed countries for a long time, as a result of the intrinsic epidemiological characteris tics of this disease, combined with improvement in the standard of liv ing and more recently the use of antibiotics. In these low prevalence countries, decisions concerning the objectives of tuberculosis program mes have to be taken and the consequences of short term changes in the sanitary situation have to be assessed. Method. A deterministic model , without age structure, of the dynamics of pulmonary tuberculosis is proposed. The model extends that of Waaler and is intended to be more suitable for application to developed countries. The flows between sev en subgroups of population, based on the natural history of the diseas e, are modelled and vaccination is taken into account. Values of model parameters and initial prevalences were deduced from published data. Results. As a first step, qualitative comparisons are performed betwee n the model-predicted decline in the annual risk of infection (ARI) an d data from the Netherlands tuberculosis survey. Using parameter value s suited to France, our model shows that the predicted decline is slow er in France than in the Netherlands; a result which tallies with epid emiological observations. Uses of the model as a decision tool are ill ustrated in two cases, that of ending systematic BCG vaccination and t hat of a sudden increase in the number of infectious cases.