This flu season is ramping up earlier than last year's.
The Centers for Disease Control and Prevention said Friday that the flu is widespread in 29 of the 54 states and territories that it tracks. This time last year, it was widespread in only four.
Eleven children have died so far this year, the CDC said. Pregnant women and the elderly are also especially vulnerable.
Federal health officials warned earlier this month that much of the influenza virus circulating in the United States this year has mutated, and that this year's vaccine does not provide much protection.
Vaccines are cooked up months in advance, and flu viruses often mutate. Flu experts say that a flu shot is still the best way to protect yourself—there are many different flu strains covered.
"Influenza vaccination is still the best way to prevent the flu—the vaccine is safe and flu kills," Dr. Carol J. Baker, executive director for the Center for Awareness and Research at Texas Children's Hospital, told NBC News.
An average of about 23,000 people die of the flu each year, although the number varies widely depending on the length and severity of the flu season.