A mathematical analysis of the spread of HIV/AIDS in Japan.

The spread of HIV/AIDS in Japan was analysed using a mathematical model incorporating pair formations between adults and sexual contacts with commercial sex workers. The parameters involved in the model were carefully specified as realistically as possible to the actual situation in Japan. Plausible ranges were assigned to those parameters for which values are not known precisely. The model was used to simulate the effect of HIV infected commercial sex workers introduced into a population without HIV. It was shown that the model could generate different scenarios, an explosive infection or a temporal spread, according to different settings of the parameters. Then the condition for occasional introduction of HIV infected commercial sex workers to be able to cause an explosive spread of HIV infection was analysed. This condition was summarized in terms of the critical transmission probability so that we could easily evaluate the degree of the risk. For some unclear parameters, sensitivity to the critical transmission probability was calculated. We also calculated a plausible range of the critical transmission probability using the Latin hypercube sampling method where the parameters were distributed on the plausible ranges. According to the analyses of the model it is concluded that the actual situation of HIV spread in Japan should lie very near the critical point that determines whether the explosive HIV spread actually takes place. This also suggests that effective action taken immediately could be useful to prevent explosive HIV infection in Japan.