Like the 1918 flu, which first appeared in the U.S. summer, the H1N1 swine flu arrived in the spring, well before traditional flu season.
And like the 1918 flu, the H1N1 flu currently seems most likely to infect older children and young adults, notes Lone Simonsen, PhD, adjunct professor of global health at George Washington University, Washington, D.C.
"What we see is the new H1N1 flu is not dissimilar to that first wave of 1918 -- which had so few deaths it was a mild pandemic wave," Simonsen tells WebMD. "That is the worst-case scenario, that we might be looking at something similar to that. Or it could be this new flu is something we have never seen before and nothing bad will follow. We cannot learn from the past exactly."
Simonsen and colleagues have studied past flu pandemics. A report on their findings appears in an early release issue of the New England Journal of Medicine.
The researchers show that flu pandemics hit different places with different severity at different times. For example, the first wave of the 1968 H3N2 flu pandemic was severe in the U.S. and Canada, but was mild in England -- which experienced a severe second wave in the winter of 1969.
Another feature of flu pandemics is that they don't happen all at once. The 1957 H2N2 pandemic is a case in point. It hit the U.S. in three waves, with most deaths occurring in 1959 and 1962 -- the last wave five years from the initial 1957 wave.
"The way the world thinks of flu pandemics is like a tornado: It swipes though and maybe it's too late to make a vaccine," Simonsen says. "But that is not true. Sometimes considerable burden falls on later flu seasons."