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Kids' Immune Systems Unfamiliar With Flu Strain

Researcher speculates why this season has been rough on children

WEDNESDAY, Jan. 14, 2004 (HealthDayNews) -- A leading influenza expert speculates that this winter's deadly outbreak has been hard on young children because it contains a strain their immature immune systems haven't encountered before.

At last count, the flu had killed 93 children, and the expert suspects many of them fell victim to a strain that was last seen in this country before they were born.

But this season also may have one good effect, says another researcher trying to keep a step ahead of the influenza virus's shifting nature: increased awareness of the need to get flu shots.

Influenza kills an average of 51,000 Americans and makes many more ill each year, Dr. John Treanor, director of the University of Rochester Medical Center Vaccine Evaluation Unit, writes in the Jan. 15 issue of the New England Journal of Medicine. It hits old people and children the hardest, he notes.

"What is perhaps more noteworthy than this year's epidemic is the extent to which we routinely accept these vaccine-preventable deaths without much comment," Treanor says.

The effort to produce effective vaccines is endless, Treanor writes, because new versions of the virus emerge continuously from waterfowl to infect humans.

These viruses contain differing forms of two essential proteins -- hemagglutinin, which attaches the virus to cells, and neuraminidase, which lets the virus spread from cell to cell. Scientists have identified 15 forms of hemagglutinin and nine forms of neuraminidase in waterfowl.

To complicate matters, the flu virus comes in two versions, type A and type B. Type A has been the dominant form in this year's outbreak. More specifically, this season's dominant strain is A/Fujian/411/2002 (H3N2), and viruses of what Treanor calls the H3 subtype are "often accompanied by the relatively early onset of a more severe influenza epidemic," he writes.

"There has been relatively little H3 virus activity in the United States for the past three years," Treanor continues. "There is therefore a relatively large cohort of susceptible children, which is probably playing an important role in transmission. In fact, the most dramatic reports of severe disease this year have been those of influenza in young children."

Each year, the U.S. Centers for Disease Control and Prevention alters the vaccine slightly, using knowledge about the probable evolution of the virus.

"You look at the evolutionary tree, then ask how the virus migrates from one pool to the next," explains Walter Finch, a professor of ecology and evolutionary biology at the University of California at Irvine who is working on the type B virus.

"I am looking at type B to see if we could predict its evolutionary course as well as is possible for the A virus," Finch says. "It requires a lot of data to do that."

While there are "lots of caveats," Finch says, "there are lots of things to be hopeful about."

The best vaccine is of no use if vulnerable people don't get it, Treanor says, so the current outbreak "may help us deal with influenza outbreaks more effectively."

Reports of the toll flu is taking on children have increased the demand for flu shots, he says, and "increased demand for vaccine will encourage manufacturers to continue producing it, possible in greater quantities."

"Vaccination coverage in this country has improved, but we still have a long way to go," Treanor says. He hopes for a cycle in which "we see increased demand for flu shots stimulate manufacturers to make more for next year."

It can be a tricky issue for pharmaceutical companies, Treanor acknowledges. "They don't want to make so much that a lot will be left over. Someone has to make some very educated guesses."

Ongoing research and large-scale production are needed because sooner or later there will be a major change in the virus that will cause a worldwide epidemic affecting many millions of people all over the world, Treanor says. New strains of the type A virus caused the influenza pandemics of the past century, most notably the Spanish flu in 1918, Asian flu in 1957, and Hong Kong flu in 1968.

More information

The whys and wherefores of influenza and the flu vaccine can be found at the U.S. Centers for Disease Control and Prevention or the American Lung Association.

SOURCES: John Treanor, M.D., professor of medicine and director, University of Rochester Medical Center Vaccine Evaluation Unit, Rochester, N.Y.; Walter Finch, Ph.D., professor, ecology and evolutionary biology, University of California, Irvine; Jan. 15, 2004, New England Journal of Medicine
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