WHO The new H1N1 flu virus could still mutate into a more virulent form..
The 1918 influenza pandemic -- which killed tens of millions of people -- began mild and returned within six months in a much more lethal form, it said. The 1968 pandemic began relatively mild, with sporadic cases prior to the first wave, and remained mild in its second wave in most, but not all countries, it said.
Seasonal flu has a so-called "secondary attack rate" -- the percentage of contacts who catch it from an infected person -- of between 5 and 15 percent, according to the WHO. Current estimates of the secondary attack rate of H1N1 range from 22 to 33 percent.
The overall severity of a pandemic is further influenced by the tendency of pandemics to encircle the globe in at least two, sometimes three, waves.. [likely more given interconnection]
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