Study uncovers cause of flu epidemics(data the researchers constructed phyloge...),Health

Big differences in the shapes of these eight trees signified that reassortment events had occurred.

The swapping of genes between two closely related strains of the influenza A virus through reassortment may also have caused the 1951 epidemic, which looked and acted in many ways like a pandemic as well. Deaths in the United Kingdom and Canada from this epidemic exceeded those from the 1957 and 1968 pandemics.

Currently, there are many types of influenza virus that circulate only in birds, which are natural viral reservoirs. Though the viruses do not seem to cause severe disease symptoms in birds, so far three of these viral types have infected humans H1N1, H2N2, and H3N2.

Understanding how each strain evolves over time is crucial. H3N2 is the dominant strain and evolves much more rapidly than H1N1. So the H1N1 component of each years flu vaccine has to be updated less often. In comparison, the H3N2 component of the vaccine has been changed four times over the past seven years.

Last year the infections were dominated by H1N1 but we had no way of predicting it, said Nelson. This year the vaccine failure is due to the H3N2 mismatch because researchers picked the wrong strain.

The H1N1 virus is particularly unusual because it disappeared completely in 1957, only to mysteriously re-emerge in humans in 1977 in exactly the same form in which it had left. It is still not certain what happened to the virus during its disappearance. But since it did not evolve at all over these twenty years, the only plausible explanation is that it was some kind of a lab escape, says Nelson, who is also affiliated with Penn States Center for Infectious Disease Dynamics (CIDD).

In recent decades, flu infections in the United States have been dominated by yet another reassorted viral str
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