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.
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.
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
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.
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.