HIV Transmission Rates Show First Infections Matter For Spread Of Virus Subtypes

HIV-1 crossed the species barrier from chimpanzees to humans roughly one century ago. While the African epicenter of the HIV pandemic is home to an array of virus subtypes, local epidemics in other regions of the world are typically dominated by a single variant, with different subtypes found in the different regions. A new study from Eötvös Loránd University in Hungary reveals most HIV epidemics are still dominated by some original strain, a "founder" virus that first entered a particular population and then blocked the introduction of alternative virus strains.

“The fast expansion of subtypes benefitted from a 'first comer advantage,' and founder viruses may have been selected by random sampling, rather than due to superior transmissibility/fitness,” the authors wrote in the introduction to their study. “However, the future evolution of the pandemic is likely to be characterized by a slow expansion of viral strains with increased transmission potential.”

The researchers began their study by using data acquired from generalized heterosexual epidemics in Africa to develop a model of sexually transmitted HIV epidemics. A model was necessary for a number of reasons. First, different HIV strains have varying rates of transmission, so a model allowed the researchers to monitor the competition dynamics of distinct strains. Second, in sexually transmitted epidemics, HIV extends through networks of sexual contacts, which are limited, by nature, since each person does not come into contact with every single other person.

To be able to monitor transmission, then, the researchers implemented a simulation that allowed them to model virus transmission along the links of separate, but interlocked networks.

HIV infection Founder effects Ferdinandy et al.

What did they observe? Once a strain of HIV had established a stable epidemic, it would slow down the invasion of secondary strains into the population. A person infected with the first HIV strain, the researchers demonstrated, are resilient to superinfection from a second strain. Because each infected person survives for a relatively long time, they impose what is effectively a "roadblock" preventing the spread of invader strains within his particular network of sexual contacts.  In this way, the first successful founder strain ruins the chances of subsequent strains gaining a foothold.

The researchers warn that the current pandemic is not static, as more transmissible strains are likely to exist or be created by mutation and recombination. Eventually, these strains may outgrow the current variants.

Source: Ferdinandy B, Mones E, Vicsek T, Muller V. HIV Competition Dynamics over Sexual Networks: First Comer Advantage Conserves Founder Effects. PLOS Computational Biology. 2015.

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