Why swine flu spared the very young and the very old

Scientists have sussed out how the H1N1 pandemic struck mostly the youthful, middle-aged population with the healthiest immune systems.
Written by Janet Fang, Contributor

If you’ll recall, the 2009 H1N1 ‘swine flu’ pandemic was peculiar because it hit healthy, middle-aged adults the hardest – unlike seasonal influenza, which strike young children who lack previous flu exposure and the elderly who generally have weaker immune systems.

And scientists have finally puzzled out why: their immune systems’ programming backfired, says study author Fernando Polack of Vanderbilt University.

People in their late teens through their 50s have been exposed repeatedly to seasonal flu viruses, which produced antibodies that circulate in the blood, fighting off new invaders that resemble previous infections.

(This explains why many elderly people were spared from swine flu last year – they were already fortified with antibodies against an H1N1 virus that circulated over 50 years ago.)

So an international team of scientists sought to figure out why children with naïve immune systems had milder swine flu symptoms.

After examining 75 severe H1N1 cases in Argentinians between ages 17 and 57, the researchers found that these patients had pre-existing antibodies against seasonal flu virus strains that ‘cross-reacted’ with the H1N1 strain.

They helped combat seasonal flu but were maladapted to mobilize against this new threat. They recognized the H1N1 virus as an invader, but rather than attack it, they assaulted organ tissue instead.

These antibodies attached to the pathogens but couldn’t stop the virus from replicating. Instead, they formed ‘pathogenic immune complexes’ rather than virus-fighting ones. The dysfunctional complexes infiltrated the lungs and triggered a biological chain reaction – rupturing cells and destroying tissue.

The immune system’s overreaction is “kind of a last-ditch way of handling something,” says epidemiologist Thomas Reichert. “If we can’t identify you specifically enough, but we know there’s a lot of you, we’re just going to blow the whole damn place up.”

The thousands of people who died were, unfortunately, victims of their own defense systems, and any universal flu vaccine will need to account for mismatched antibodies.

Last month alone, Walgreens administered about a million flu shots, and their total sales grew 8.6 percent in November to $5.83 billion.

The study appeared Sunday in Nature Medicine.

Image: CDC

This post was originally published on Smartplanet.com

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