Every year influenza contributes to the death of 20,000 people in the U.S. and perhaps millions worldwide. The virus rides into your body on an inhaled water droplet, then tunnels into your cells, replicates, and invades other cells. Your immune system can hunt down and kill the organisms, but it takes a week or more. The spreading virus can overwhelm a person whose immune system does not respond strongly or quickly enough, leading to complications such as pneumonia.

Vaccination provides a training exercise that teaches the immune system how to muster a swift counterattack. Because the virus mutates regularly, a new vaccine must be mixed for each flu season, which in the U.S. usually begins by November and peaks by February. During the previous winter, the World Health Organization recommends three flu strains for the upcoming year's cocktail for the Northern Hemisphere, based on which strains are surging. The 2000¿2001 U.S. recipe includes antigens (immunological targets) from A/Panama, A/New Caledonia and B/Yamanashi strains. By February the Food and Drug Administration or the Centers for Disease Control and Prevention provide viral stock to U.S. pharmaceutical companies. The firms inject the stock into fertilized chicken eggs, where it reproduces. They grow each strain separately and then draw off allantoic fluid (egg white) to harvest the virus, purify it, inactivate it, blend the strains with a carrier fluid, and dispense that into vials. Production is largely finished by August, and shipment to health organizations is completed by October.

This article was originally published with the title Preparing for Battle.

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