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Swine Flu: Learning From Past Pandemics

New Flu Shares Features of Deadly Flu Pandemics

By Daniel J. DeNoon
WebMD Health News

Reviewed By Louise Chang, MD

May 8, 2009 -- The H1N1 swine flu outbreak has a lot in common with flu pandemics of the 20th century -- including its direct ancestor, the killer 1918 H1N1 flu.

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H1N1 Swine Flu

Like the first wave of the 1918 flu, the U.S. outbreak of H1N1 swine flu has been relatively mild.

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."

And sometimes that burden doesn't come during flu season at all. That's an important warning, as a poll shows Americans are looking past the current H1N1 outbreak to worry about the looming fall flu season.

It isn't a bad plan to prepare for the coming flu season, notes Carolyn Bridges, MD, of the CDC's epidemic intelligence service.

"In past pandemics with a novel flu strain, the initial outbreaks in the summer are generally milder," she said at a news conference. We expect we are likely to see some transmission over the summer, with the possibility that over the fall, when the weather turns cooler, we might see an increase in cases."

But it isn't a good idea to take the current spring outbreak lightly. Simonsen notes that the second, more severe wave of the 1889 flu pandemic swept through London in the summer months. And the 1957 flu hit the U.S. in September while there was still summer weather.

"These pandemics violate one rule after another that we think we know about flu," Simonsen says.

This fact isn't lost on the CDC.

"That is always the tricky part with influenza: You never know what we'll get until we get there," Bridges says.

Based on history, Simonsen says there are three main things to keep an eye on:

  • Get a really good grip on the age distribution of severe outcomes, especially deaths, to learn which populations most need protection from antiviral drugs and vaccines.
  • Test lots of people for H1N1 swine flu antibodies, so that a case-fatality rate can be calculated.
  • Watch what happens in the Southern Hemisphere during its flu season.

Richard Besser, MD, acting CDC director, said at a news conference today that the CDC is actively pursuing all of these studies -- including keeping an eye on the Southern Hemisphere.

"That will tell us whether virus could return here in a form dangerous to human health," Besser said. "And if this were to come back in more severe form, we are looking to see what do we need to be able to do in our communities and at the federal level. As individuals ... think about how to make sure you are ready."

Planning ahead does not mean turning a blind eye to the present.

"Don't take your guard off things you can do right now," Besser said. "Because we are hearing from states of increased flu activity."

SOURCES: Miller, M.A. New England Journal of Medicine, published online ahead of print, May 7, 2009. Lone Simonsen, PhD, adjunct professor of global health, School of Public Health and Health Services, George Washington University, Washington, D.C. Carolyn Bridges, MD, epidemic intelligence service program, CDC, Atlanta. Richard Besser, MD, acting director, CDC, Atlanta.

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