As health officials brace for a new onslaught of illness from the novel H1N1 virus, they remain perplexed by one of the most unusual and unsettling patterns to emerge from this pandemic -- the tendency of the so-called swine flu to strike younger, healthier people.
The initial explanation was that the elderly, who are usually most vulnerable to the flu, have built-in immunity as a result of their exposure more than 50 years ago to ancestors of today's pandemic strain. But the limits of the theory are becoming more clear. For starters, only a third actually have antibodies to the new H1N1.
"It doesn't quite look as though it's the whole story," said Dr. William Schaffner, chairman of the department of preventive medicine at Vanderbilt University School of Medicine in Nashville.
Further, the flu's two key genes came directly from pigs and are new to everyone. That means all age groups should be equally vulnerable since no one has encountered the genes before. Yet infants seem to be in less danger than older children and most adults.
Unraveling these mysteries will be crucial to designing a strong defense against this tenacious virus. Tests in animals strongly suggest that H1N1 will be with us for the foreseeable future, supplanting the strains that cause seasonal flu within a year or two.
Understanding the virus' inner workings will also help scientists prepare for future influenza pandemics.
H1N1's unorthodox nature became apparent as soon as the virus burst onto the scene in early spring: Most of the earliest cases in the U.S. and Mexico occurred in children rather than the elderly.
In the U.S., half of the people with confirmed H1N1 cases have been 12 or younger, according to the Centers for Disease Control and Prevention. Until July 24, when the CDC stopped counting new cases, 60% of patients whose ages were known were between 5 and 24 years old. An additional 20% were in the 25-to-49 age group. Only 1% of those sickened had reached the age of 65.
Younger people also appear to be getting the most severe cases of pandemic flu. The median age of patients requiring hospitalization is 20, according to the CDC. In fatal cases, the median age is 37.
Those figures are in stark contrast to the seasonal flu, which is most likely to sicken people who are elderly, very young or chronically ill. More than 90% of fatal cases involve senior citizens.
H1N1's link to 1918
