How exactly do new influenza gene teams make the leap from aquatic birds to a new host, such as people or other mammals? What factors determine whether infection in a new host yields a dead-end infection or sustained, human-to-human transmission, as happened in 1918? Research on such topics is intense, but at this time definitive answers remain elusive, notes Dr. Morens.
It is known that the human immune system mounts a defense against the influenza virus's H and N proteins, primarily in the form of antibodies. But as population-wide immunity to any new variant of flu arises, the virus reacts by changing in large and small ways that make it more difficult for antibodies to recognize it. For nearly a century, then, the immune system has been engaged in a complicated pas de deux
with the 1918 influenza virus and its progeny, say the NIAID authors. The partners in this dance are linked in an endless effort to take the lead from the other.
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While the dynasty founded by the virus of 1918 shows little evidence of being overthrown, the NIAID authors note that there may be some cause for optimism. When viewed through a long lens of many decades, it does appear that successive pandemics and outbreaks caused by later generations of the 1918 influenza dynasty are decreasing in severity, notes Dr. Morens. This is due in part to advances in medicine and public health measures, he says, but this trend also may reflect viral evolutionary pathways that favor increases in the virus's ability to spread from host to host, combined with decreases in its tendency to kill those hosts.
"Although we must be prepared to deal with the possibility of a new and clinically severe influenza pandemic caused by an entirely new virus, we must also understand in greater depth, and continue to explore, the determinants and dynamics of the pandemic era in which we live," conclude the authors.
(See a diagram of the genetic relationships among human and swine influenza viruses at http://www3.niaid.nih.gov/news/newsreleases/2009/flu_genetic_lineage.htm.)
Source: The persistent legacy of the 1918 influenza virus. New England Journal of Medicine. DOI: 10.1056/NEJMp0904819 (2009)
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