Washington (CNN) -- As students return to U.S. classrooms, it's time for people to begin thinking about flu shots.
Every year between 5 percent and 20 percent of the U.S. population gets the flu, the Centers for Disease Control and Prevention says. The U.S. Department of Health and Human Services recommends that everyone age 6 months or older be vaccinated.
Manufacturers began shipping this year's flu vaccine at the end of July. Eventually 155 million doses should be available for the coming flu season.
As always, the vaccine should protect against three flu viruses. This year's vaccine is formulated for an H3N2 virus, an influenza B virus and the H1N1 virus that grabbed headlines last season.
On August 10, the World Health Organization declared an end to the H1N1 pandemic but warned that this doesn't mean that the H1N1 virus no longer exists. Instead, it means that the virus has moved to an intensity similar to that seen during seasonal epidemics.
During the fall of last year, H1N1 was responsible for closing at least 1,947 schools across the United States, impacting more than 600,000 students, officials said.
CNN's Sally Holland contributed to this report.