Tracking the Wild Flu

By C. CLAIBORNE RAY

. How do scientists formulate each year's flu shot?

A. Scientists try to choose a cocktail of viruses that reflect the strains that are likely to be of concern in the coming flu season. They base their decision on monitoring and educated guesswork about how the strains will mutate.

The monitoring is done by laboratories in about 80 nations under the auspices of the World Health Organization. It is coordinated by major centers in Atlanta (the Centers for Disease Control and Prevention), London, Melbourne and Tokyo.

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The centers isolate and test samples to find out their genetic makeup. Then W.H.O. recommends strains that seem to be prevalent, or related strains that may be easier to culture and manufacture. Public and private experts meet in the winter to make the final choice.

Each new strain of influenza produces new antigens, the proteins that elicit the body response that creates immunization.

There are two main types of influenza, A and B, and the vaccine usually includes two A strains and one B strain.

The formula for flu shots being given now was announced on March 6 by the Food and Drug Administration. As usual, it follows the recommendations of the World Health Organization. It consists of strains designated as H1N1, A/New Caledonia/20/99; H3N2, A/Panama/2007/99 (an A/Moscow/10/99-like virus); and B/Hong Kong/330/2001-like virus strain.