Sexual transmission of HIV by acutely infected individuals has a disproportionate effect on the spread of HIV and may explain the current pandemic

A large number of observational studies has estimated the average probability of male-female transmission of HIV per unprotected coital act to be 1/384—1/2000 transmission events per coital act during established (ie no-acute) HIV infection.

In a study using survey-based data on sexual behaviours in the United States, Pinkerton et al calculated that these probabilities of transmission per coital act would result in low rates of lifetime transmission (0.190.40 infected partners/man; 0.090.18 infected partners/woman), which, by themselves, could not sustain an epidemic.

The fact that genital fluids (not blood) are the principal vehicles for sexual transmission of HIV presents a particular problem for modeling the likelihood of HIV transmission during acute HIV infection on the basis of blood data. This is because acute HIV infection represents the period of initial establishment of anatomic HIV reservoirs; therefore, the viral dynamics in blood, which have been well described for acute HIV infection, cannot be assumed to apply to the genital tract.

Researchers in the Quest Study Group and the Duke-Emory Acute HIV Consortium examined whether viral dynamics in the genital tract during the natural history of acute HIV-1 infection could explain efficient heterosexual transmission of HIV. The investigators measured HIV concentration in blood and semen samples from patients with acute and long-term HIV infection. They than explored the effect of changes in viral dynamics in semen on the probability of transmission per coital act, using a probabilistic model published elsewhere.

Considered over time from infection, semen HIV concentrations, in men with acute infection, increase and decrease in approximate parallel with changes occurring in blood. Modeling suggests that these acute dynamics alone are sufficient to increase probability of heterosexual transmission by 810-fold between peak (day 20 after infection, based on the model) and virologic set points (day 54 and later after infection). Depending on the frequency of coitus, men with average semen HIV loads and without sexually transmitted diseases (STDs) would be expected to infect 7%-24% of susceptible female sex partners during the first 2 months of infection. The predicted infection rate would be much higher when either partner has an STD. The authors conclude: “Empirical biological data strongly support the hypothesis that sexual transmission by acutely infected individuals has a disproportionate effect on the spread of HIV infection. Acute hyperinfectiousness may, in part, explain the current pandemic in heterosexual individuals.”

The present study provides empirical evidence that men with acute HIV infection are biologically hyper-infectious because of increased genital shedding of HIV. In addition, the present study has provided evidence that, during acute infection, HIV load increases and decreases in semen in approximate parallel with changes occurring in blood, which have been well described. The present model of viral dynamics in semen suggests that, on average, individuals are hyper-infectious beginning before the onset of the acute retroviral syndrome and continuing for approximately 6 weeks thereafter.

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Pilcher CD, Tien HC, Eron JJ Jr et al. Brief but efficient: acute HIV infection and the sexual transmission of HIV. J Infect Dis. 2004 May 15;189(10):1785-92. Epub 2004 Apr 28.