It's still not too late to get a flu shot, the U.S. Food and Drug Administration said Wednesday.
Flu activity often peaks in January or February and can last well into May, and a flu shot protects you as long as flu viruses are circulating, the FDA said.
Children and seniors tend to be most susceptible to flu. But sometimes a flu virus will affect more young and middle-aged adults. That appears to be the case this flu season, the agency said.
An unusually high number of severe respiratory illness in young and middle-aged Americans were reported to the U.S. Centers for Disease Control and Prevention in November and December, the FDA said in a news release.
Many of those cases were caused by the H1N1 "swine flu" strain that affected more children and young adults than older adults during the 2009 pandemic. Protection against the 2009 H1N1 virus, which has circulated each year since the pandemic, is included in this year's vaccine, the FDA said.
"Influenza seasons and severity are often unpredictable. Annual influenza vaccination is the best way to prevent influenza among people 6 months of age and older," Marion Gruber, director of the FDA's Office of Vaccine Research and Review, said in the news release.
"However, taking such practical measures as washing hands, covering coughs and sneezes and staying home when sick can also help to decrease the spread and minimize the effects of flu," she added.
Antiviral drugs, such as Tamiflu, aren't a substitute for a flu shot, but they can help treat the flu, according to the FDA.
New flu vaccines need to be produced every year because different subtypes and strains of flu viruses circulate each season, Gruber explained.
"The closer the match between the circulating strains causing disease and the virus strains in the vaccine, the better the protection against influenza," she said.
Federal health officials have previously reported that this year's vaccine is a good match for the circulating viruses.
