2. Know Your EnemyEvery year between 5 and 20 percent of the U.S. population comes down with seasonal influenza. You cannot acquire lifelong immunity, as you can with polio or measles, because flu viruses constantly mutate into new strains. Of those that infect humans, Type A is the most virulent. It kills 36,000 Americans a year and puts another 164,000 in the hospital. Type B is more benign but still brutal, while Type C is like a mild cold.
Producing a vaccine to combat such a variable pathogen is like hitting a moving target. Health officials confer almost a year in advance, near the height of each hemisphere’s flu season (February up north, September down south). The vaccine contains three inactivated viruses—two Type A strains and one Type B—that trigger your immune system to produce antibodies against those particular viruses. Based on the most common strains going around that year, officials estimate which are likely to hit hardest the following season and include those three in the vaccine. "It’s a roulette game," says David Taylor, M.D., chief medical officer of biotechnology company VaxInnate. Even so, "It’s still worthwhile to get a shot," says Carolyn Bridges, M.D., of the Centers for Disease Control, because it offers cross-protection and reduces the severity of individual cases.
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