Each flu season is a new adventure for public health experts, since the viruses that cause the flu are always changing. "Every flu season is different, and we don't know ahead of time when we're going to start to see flu circulation," Melissa Stockwell, a pediatrics specialist at Columbia University’s medical and public health schools, told Newsweek.
Because the vaccine takes two weeks to protect its recipient—and three weeks is more effective—health officials and the public need to stay ahead of the virus. The CDC bases its recommendations on when flu infections are likely to become common, this year recommending people aim to get the vaccine by the end of October. "We use the end of October because in general, in most seasons, things have begun to pick up by then," said Lisa Grohskopf, an epidemiologist at the CDC and lead author on this year’s vaccine recommendations.
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