Flu Super Strains Could Emerge Decades After Their Disappearance

Influenza super-strains can emerge through recombination of strains from birds, pigs, and humans. However, once a new recombinant strain emerges, it is not clear whether the strain is capable of sustaining an outbreak. In certain cases, such strains have caused major influenza pandemics. Brian J. Coburn, of the Center for Biomedical Modeling at the David Geffen School of Medicine, University of California, Los Angeles, and colleagues, developed a multi-host (i.e., birds, pigs, and humans) and multi-strain model of influenza to analyze the outcome of emergent strains. In the model, pigs acted as "mixing vessels" for avian and human strains and can produce super-strains from genetic recombination.

The researchers say that epidemiological outcomes are predicted by three factors: contact between pigs and humans, transmissibility of the super-strain in humans, and transmissibility from pigs to humans. Specifically, outbreaks will reoccur when the super-strain intections are less frequent between humans (e.g., R0=1.4) but grequent from pigs to humans, and a large-scale outbreak followed by successively damping outbreaks will occur when human transmissibility is high (e.g., R0=2.3). The average time between the initial outbreak and the first resurgence varies from 41 to 82 years. Coburn, et al. determined the largest outbreak will occur when 2.3 <R0 < 3.8 and the highest cumulative infections occur when 0 <R0 < 3.0 and is dependent on the frequency of pig-to-human infections for lower R0 values (0 <R0 < 1.9).

The researchers say their results provide insights on the effect of species interactions on the dynamics of influenza super-strains. Counter intuitively, epidemics may occur in humans even if the transmissibility of a super-strain is low. They add that surprisingly, their modeling shows strains that have generated past epidemics (e.g., H1N1) could resurge decades after they have apparently disappeared. Their research was published in BMC Public Health.

Reference: Coburn BJ, Cosner C and Ruan S. Emergence and dynamics of influenza super-strains. BMC Public Health 2011, 11 (Suppl 1):S6doi:10.1186/1471-2458-11-S1-S6