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The Peak Argument

The second argument is that the epidemic is already under control. This is based on the declining rate of reports of new AIDS cases.

Models that predict that the transmission of HIV has peaked are premised on the assumption that AIDS cases are an effective index of HIV transmission. (They also assume that most AIDS cases are reported, which is not supported by death certificate studies.) The long latency of infection and the dramatically different dynamics of spread in most heterosexual groups lead to an underestimate of the latency of spread into the general community. Models based on back calculation of infection rates from AIDS case reports are sensitive to the rate of spread in the community being modeled and the latency of the disease. If the latency is longer than the model assumes, the model underestimates the infection rate--and the lower infection rate lowers the predicted equilibrium rate of infection, the point at which new cases equal deaths or cures. If the disease spreads fast enough and kills quick enough, it can go to extinction: it consumes the susceptible population.

Overestimating the rate of transmission also lowers the predicted equilibrium rate of infection. This happens because the time to reach equilibrium is dependent on the size of the population modeled, the percentage of infected persons at the point when the prediction is made, and the rate of transmission. Other things held constant, the faster the rate of infection is, the faster the system reaches equilibrium. Thus, at a given time after the introduction of the disease into the community, the faster the rate of transmission is, the closer the disease is to equilibrium. This is mathematically the same problem as chemical reactions: the faster the reaction is, the sooner the reaction is at equilibrium.

The high rate of sexual activity among gay men frequenting bathhouses resulted in a high rate of transmission, and the disease rapidly reached equilibrium in the community. Conversely, the slower rate of transmission of HIV in the heterosexual community means that current levels of AIDS cases are further from the equilibrium levels. If the models of heterosexual transmission overestimate the rate of transmission, they will underestimate the time to reach equilibrium. This leads to the conclusion that current levels of infection are closer to equilibrium levels than would be the case if a lower transmission rate was used.

The confounding factor in heterosexual transmission is that the heterosexual population is multimodal: there are several subgroups of heterosexuals with substantially different transmission rates, with approximately 10 percent of persons having a much higher rate of transmission than the remaining population. This has been well established in models of other sexually transmitted diseases.[145] It is reasonable to assume that current AIDS cases in heterosexuals disproportionately represent transmission in the 10 percent with high rates of sexual activity. If this is the case, the rate for the remaining 90 percent of the heterosexual community is much lower than has been assumed in the models of HIV transmission.

This lower rate of transmission of HIV in heterosexuals means that the predicted equilibrium rates will be much higher and will take much longer to achieve. If this analysis is correct, the rate of new AIDS cases in heterosexuals should slowly increase until at least the year 2000. Ultimately, however, this low rate of transmission will limit the equilibrium level in heterosexuals because of the low infectivity of HIV. Outside the highly sexually active subgroup, the average number of lifetime contacts with an infected person will be low enough to limit the transmission of HIV among heterosexuals to much lower levels than the 50 percent or greater that were seen in highly sexual active gay men.

[145]Hethcote HW; Yorke JA: Gonorrhea Transmission Dynamics and Control. 1984.

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