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Part 4: How are Flu Vaccines Made?

To best understand the material in this web article, read the sections in our textbook that explain how the immune system works. (Dragonfly book Sections 40-1 and 40-2; Elephant book sections 45-1 and 45-2.)

Each year in January, production lines gear up to produce between 12 and 30 million doses of flu vaccine — each designed to protect against three different flu strains. The tough part is deciding which three strains to include. Not only must vaccine makers judge which strains are different enough to trouble the immune system, they must also determine which strains spread most easily. In making those decisions, flu experts make scientifically-based predictions that are literally matters of life and death.

On what data are those predictions based? The Center for Disease Control in Atlanta constantly receives virus samples from all over the world. Each sample is logged into a database that records where the strain came from, whether it caused serious illness, and whether it infected only a few isolated people or seemed to be spreading rapidly. Each strain is genetically fingerprinted and compared with known strains. Parts of the original samples are stored in a deep freeze, and parts are inoculated into chicken eggs or special cell cultures to be grown for further study.

Samples whose coat proteins look "new" enough to cause trouble are tested to see how infectious they are, and how long it takes the immune system to respond to them. Throughout the entire process, the cultured strains are monitored to make certain that they haven't mutated too much from the original samples. In the meantime, researchers in the lab stay in constant contact with public health officials and fieldworkers in the parts of the world where the most "interesting" strains originated. Why? Because, for reasons that still arenšt understood, some strains spread quickly in human populations, while others just don't get around very much.

The results of all these studies are combined, analyzed, and presented to expert panels at a meeting of the U.S. Food and Drug Administration (FDA) Vaccines and Related Biological Products Advisory Committee. Then the debates begin. Which strains produce the most serious symptoms? Which spreads from person to person most effectively? Which strains seem to be emerging from the areas in which they first appeared? Which are staying put? Which of the potentially serious strains can be grown well enough in the lab to produce enough vaccine?

Finally, the experts use these data to make the best predictions they can about which three strains are most likely to cause the most trouble in the United States the following winter.

Once the decision is made, the rush to manufacture vaccine is on. For the present, vaccines are still grown in an "old fashioned" way. Chosen viral strains are inoculated into 90 million fertilized chicken eggs in which the viruses are incubated. The eggs are opened, and the virus is extracted and purified. Then the virus is treated chemically to kill it while preserving the normal shape of its coat proteins. Each final dose of vaccine is very small, containing scarcely fifteen millionths of a gram of each of three different viral strains. Yet that tiny amount of foreign protein, injected into your body, is enough to place your entire immune system on red alert. Assuming that you are vaccinated at least a week before you are exposed to live virus, your B-cells and T-cells will be ready to defend you — but only against the strains contained in the vaccine.



Index of this Web Article:

• Introduction — What's up with the Flu?
• What is influenza — and who gets it?
Why can we get the flu again and again? And why can't we develop a one-time flu vaccine?
• How are flu vaccines made?
• Why are flu vaccines sometimes not effective?
• Why are researchers so worried about this new bird flu?

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