Age of Flu Victims Has Big Implications
Scientists Say Relative Youth of Ill People Is Evidence of Pandemic Potential

By David Brown
Washington Post Staff Writer
Sunday, May 17, 2009

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. H1N1 is spreading easily in North America but sputtering in Europe, just as Hong Kong flu did in 1968. And as in the mini-pandemic of Russian flu in 1977, some people appear to have a degree of immunity.

There are two theories for what is happening.

One is that students visiting Mexico on spring break were the chief “vectors” bringing the virus to the United States, where they then infected schoolmates and friends. The other is that young people are especially vulnerable for some reason.

“As we get farther and farther in, are we going to be able to choose between these two hypotheses? Sure we are,” Joseph Bresee, the CDC’s chief flu epidemiologist, said late last week. But, he added, it may take two or three months.

Determining the true age distribution is crucial, as it will help set the policy for who should be first in line for vaccines and how to ration antiviral drugs if they are in short supply. “It really does have big implications,” Bresee said.

A closely related question is whether the illness tends to be more serious in younger age groups as well as more common.

In the United States, the familiar seasonal influenza causes about 8,100 deaths a year directly and contributes to about 36,000 more in people with lung or heart problems. Ninety percent of those deaths occur in people 65 and older. The risk of a healthy person older than 65 dying directly from flu is about 100 times that of a healthy person 5 to 49 years old.

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.