Why Did Swine Flu Kill Healthy Adults?

One of the most intriguing questions about the swine flu epidemic last year was why most of the deaths occurred in healthy young adults. Why were the very young and the very old generally spared?

A recent paper in Nature Medicine provides a clue, according to a news article in Science magazine. The gist of it is that the immune system of most adults is not very effective against first exposure to the H1N1-type virus. Unable to kill the virus initially with just a normal first immune response, the immune system in some patients mounted an all-out “do or die” effort to kill the virus. The result was a severe inflammatory reaction in the lungs that ultimately killed the patient instead of the virus.

The theory of a hyperactive but ineffective immune system would explain why the very young and the very old were spared by swine flu. The very young do not have a fully developed immune system with which to mount even a normal immune response, much less an exaggerated one. And many older persons may have had at least some effective antibodies against H1N1 by virtue of having been exposed to the previous H1N1 strain that was around until the late 1950’s.