HIV transmission by stage of infection and pattern of sexual partnerships

Abstract

Background: Most model analyses examining the role of primary HIV infection in the HIV epidemic ignore the fact that HIV is often transmitted through long-term, concurrent sexual partnerships. We sought to understand how duration and concurrency of sexual partnerships affect the role of transmissions during primary HIV infection.

Methods: We constructed a stochastic individual-based model of HIV transmission in a homogeneous population where partnerships form and dissolve. Using observed contagiousness by stage of HIV infection, the fraction of transmissions during primary HIV infection at equilibrium was examined across varying partnership durations and concurrencies.

Results: The fraction of transmissions during primary HIV infection has a U-shaped relationship with partnership duration. The fraction drops with increasing partnership duration for partnerships with shorter average duration but rises for partnerships with longer average duration. Partnership concurrency modifies this relationship. The fraction of transmissions during primary HIV infection increases with increasing partnership concurrency for partnerships with shorter average duration, but decreases for partnerships with longer average duration.

Conclusions: Partnership patterns strongly influence the transmission of HIV and do so differentially by stage of infection. Dynamic partnerships need to be taken into account to make a robust inference on the role of different stages of HIV infection.

FIGURE 1

Compartmental flows. The natural history of HIV infection is modeled with 3 stages—primary (P), asymptomatic (A), and late stages (L) of infection. The susceptible population is represented with S. The vertical arrows represent removal from the sexually active population. The leftmost horizontal arrow represents susceptible men? entering the sexually active population. The other horizontal arrows represent infection and stage progression with the final arrow being death from AIDS. Parameters are defined in Table 1. Probability of infection (in the arrow from S to P) is explained in Methods section.

FIGURE 2

A, The typical time series of a simulation run for mean degree n = 1.5, partnership dissolution probability per day σ = 0.001, and mean frequency of sex acts per partnership per day c = 0.3333. B, Prevalence of infection and infections from primary HIV infection over the first 10 years. C, Years elapsed to reach half of endemic levels under varying average partnership duration and mean degree.

FIGURE 3

Simulation results when partnership duration is varied. The mean degree n = 1.5 and the mean frequency of sex acts per partnership per day c = 0.3333. S-I and S-P pairs denote a pair of a susceptible person and any infected person, and a pair of a susceptible person and a person infected with primary HIV infection, respectively. Each data point is the mean from 10 simulation runs, and error bars indicate one standard deviation. A, Endemic prevalence and the fraction of S-I pairs out of all pairs across partnership duration. B, Fraction of transmissions during primary HIV infection and the fraction of S-P pairs out of all S-I pairs across partnership duration.

FIGURE 4

Simulation results when the mean degree n as well as partnership duration is varied. The mean frequency of sex acts per partnership per day c is varied in the way that c × n = 0.5. A, Endemic prevalence across partnership duration at 4 levels of n. B, Fraction of transmissions during primary HIV infection across partnership duration at 4 levels of n. C, Fraction of S-P pairs out of all S-I pairs across partnership duration at 4 levels of n.

FIGURE 5

Degree distribution at 4 levels of ξ. Mean degree n = 0.9.

FIGURE 6

Simulation results when ξ as well as partnership duration is varied. Mean degree n = 0.9, mean frequency of sex acts per partnership per day c = 1.1111. A, Endemic prevalence across partnership duration at 4 levels of ξ. B, Fraction of transmissions during primary HIV infection across partnership duration at 4 levels of ξ. C, Fraction of S-P pairs out of all S-I pairs across partnership duration at 4 levels of ξ.