Primary HIV infection as source of HIV transmission within steady and casual partnerships among homosexual men

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Abstract
To assess the contribution of primary or acute HIV infection to the transmission of HIV among homosexual men in Amsterdam and to investigate how the initiation of treatment during primary HIV infection (PHI) can affect the incidence of HIV infection. A mathematical model describing HIV transmission among homosexual men was developed. In the model, men are involved in both steady and casual partnerships. Infectivity is higher during PHI than during chronic HIV infection. Highly active antiretroviral therapy reduces infectivity and increases the time to the development of AIDS. Its effect is enhanced if treatment is initiated during PHI. HIV incidence and the fraction of transmission attributed to PHI were calculated for different levels of treatment efficacy. Primary infections account for 35% of HIV transmissions from casual partners and 6% of transmissions from steady partners. Among all new infections, only 11% occurs during PHI. Therefore, the effect of treatment during PHI on the incidence of HIV is limited. However, in a community with higher risky behaviour among casual partners, the fraction of transmission attributed to PHI increases to 25%. Primary infections play a more important role in transmission from casual partners than in transmission from steady partners. Therefore, in communities in which steady partners account for the majority of new infections and the epidemic is at an advanced phase, the contribution of PHI to the transmission of HIV is rather small and the effect of early treatment on the incidence of HIV is limited.