The swine-origin influenza A (H1N1) virus that burst into public consciousness a month ago is starting to behave like a mixture of its infamous, pandemic-causing predecessors.
It seems to have a predilection for young adults, as did its notorious ancestor, the 1918 Spanish influenza. Many of the young victims who have become deathly ill turned out to have other medical problems -- a phenomenon first clearly seen with the 1957 Asian flu.
Pandemic flu strains -- and this new H1N1 strain is all but certain to cause the 21st century's first pandemic -- are unpredictable. Any contagious disease that most of the world's 6.8 billion people can catch is inherently dangerous.
"Our message to everybody is, of course, do not over-worry about these things, [but] it is important to know it is serious," the WHO's Keiji Fukuda said last week.
Perhaps the most worrisome features so far are the number and severity of cases in teenagers and young adults. This was noticed early, and the pattern has not changed much now that there are 5,000 laboratory-confirmed infections and probably more than 100,000 overall. The average age of the confirmed and probable cases is 15 years. Two-thirds are younger than 18.
Compared with seasonal outbreaks, all flu pandemics cause a higher percentage of severe cases and deaths in younger groups. Although the overall mortality rate from the current swine flu is low, this trend is already apparent.
Last Thursday, when Fukuda announced that the global death total was 65, he noted that "half of them are healthy people who have no predisposing conditions. This is a pattern different from what we see with normal influenza."
There have been too few deaths in the United States to draw any conclusions. But of the 173 people who have been sick enough to be hospitalized, more than half are in the 5-to-24 age group.
