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Who's at High Risk From Swine Flu?

New Flu Riskiest for Pregnant Women, Kids Under 5 or With Asthma, Many Others

Why Do Some People Suffer Severe Swine Flu?

Many of the people who have suffered severe complications of swine flu -- including death -- have not been in any of the risk groups. Alarmingly, some have been apparently healthy young people.

One reason for this is simply statistical. Because the first wave of the swine flu pandemic has spread most explosively in schools, young people have been exposed in disproportionate numbers. Because more young people are infected, more of the severe cases (as well as more of the mild cases) occur in this population.

Another reason may have to do with the virus. While the pandemic 2009 H1N1 bug does not seem to carry any of the flu genes linked to extreme virulence, it does more readily infect lung tissues than the seasonal flu bugs now in circulation.

Ferret studies suggest that the pandemic swine flu bug isn't yet fully adapted to humans because it doesn't spread easily in small sneeze and cough droplets. That's a good thing, because small droplets carry the virus deep into the lungs. This may be why the swine flu causes relatively few severe flu cases, despite the virus's ability to home in on lung tissue.

A third reason may have to do with immunity. The seasonal flu vaccine doesn't protect against the type A H1N1 swine flu, even though it immunizes against a seasonal type A H1N1 virus. But people who have had multiple vaccinations, or who once were infected with an H1N1 virus, might mount a better immune response to swine flu than younger people with less exposure to vaccine and/or flu.

And there's evidence that people born before 1957 -- when an ancestor of the swine flu bug was in seasonal circulation -- might have some degree of protection against swine flu.

But there's no definitive answer to why some people suffer worse flu disease than others. The best course for everyone is to avoid infection when healthy and to avoid infecting others when sick.

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