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Swine Flu Pandemic FAQ

What the swine flu pandemic means to you.

Why has WHO declared a pandemic now?

WHO's technical criteria for declaring a pandemic is that the infection must be spreading locally in at least two distinct regions of the world. That actually went on for some time before the official declaration.. But the WHO was worried that governments might overreact to a pandemic declaration.

They had good reason to think so. Early in the pandemic, some countries stopped importing pork and even slaughtered local pig herds -- even though the so-called swine flu is spread from person to person and cannot be spread by eating pork. And other nations established unreasonable travel restrictions or unnecessarily quarantined healthy people from countries where the flu was spreading.

These unnecessary actions had serious economic and social impacts. And since most public health experts feared that the next flu pandemic would be the vastly more deadly H5N1 bird flu, most pandemic preparedness plans contained steps far more drastic than steps needed to fight swine flu, which is only moderate in severity.

The six-stage WHO pandemic alert system does not take disease severity into account; it's based on the geographic spread of a virus.

The CDC does have a pandemic severity scale, which has five categories. The scale is based on the percentage of infected people who die -- the case-fatality ratio.

It's too soon to know the case-fatality ratio for H1N1 swine flu. But the CDC's best estimate so far is that it is 0.1%. That puts it on the borderline between Category 1 (the lowest category) and Category 2. The 1918 swine flu was a Category 5 pandemic, with a case-fatality rate of over 2%.

Even a Category 1 pandemic is serious. The CDC estimates that a pandemic with a 0.1% case-fatality ratio would result in some 90,000 U.S. deaths if no vaccine becomes available.

Has H1N1 swine flu become more dangerous?

No. So far, the H1N1 swine flu virus that is spreading around the globe is very similar to the swine flu viruses first seen in North America.

Experts say the disease is moderate in severity. That is because most cases -- and most hospitalizations -- have been in young people 5 to 24 years old. A small proportion of these young people have died.

Even though most people who get swine flu recover fully, these troubling deaths in otherwise healthy young people make experts hesitate to call the disease "mild."

Flu viruses do, of course, mutate. They may become less dangerous. But they may become more deadly, possibly picking up virulence factors from other flu bugs circulating at the same time. The nightmare scenario is that the H1N1 swine flu would combine with the H5N1 bird flu to create a fast-spreading, lethal virus. But the chance of this happening is small.

And pandemics come in waves. They tend to appear, wane, and reappear over two or three years. Sometimes there may be a mild first wave, followed by far more serious waves. That's what happened in 1918 and 1919 -- and that's what keeps public health officials awake at night.

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