Last year's flu season was slow to start and never became a big threat — but scientists warn there's no predicting what will happen this year.
Flu shots are becoming available in doctors' offices and pharmacies. Health experts recommend everyone 6 months and older get vaccinated.
The flu shot, which contains antibodies to protect against three strains of influenza, was changed earlier this year to account for virus activity in the Southern Hemisphere.
Every year, scientists collect virus samples from around the world to predict which strains will travel north for the flu season.
The 2012-2013 flu shot will contain the 2009 H1N1 virus from past years plus new H3N2 and B strains. Last year's vaccine was considered a fairly good match to the circulating viruses.
A typical flu season lasts from October to May with a peak in February. Flu activity last season did not peak until the middle of March, with just one week of elevated flu symptoms reported from doctor visits.
The mild winter may have led to fewer people getting sick with flu, according to experts at the Centers for Disease Control and Prevention. Other likely factors include a well-matched vaccine and a lack of mutation in the virus from earlier seasons.
The different viruses expected to circulate this winter could mean the flu season will be more severe, although predictions are notoriously difficult.
The flu shot is recommended for everyone except newborns, but certain groups have a higher risk of developing complications from the flu. Pregnant women, people older than 65 and anyone with asthma or diabetes are especially encouraged to get vaccinated.
The flu vaccine is available by injections into the muscle or the skin, or through a nasal spray. The nasal spray contains a weakened version of the flu virus and is recommended only for healthy people ages 2 to 49.