One key difference between then and now: In 2003-04, the vaccine was poorly matched to the predominant flu strain. Also, there's more vaccine now, and vaccination rates have risen for the general public and for key groups such as pregnant women and health care workers.
An estimated 112 million Americans have been vaccinated so far, the CDC said. Flu vaccinations are recommended for everyone 6 months or older.
On average, about 24,000 Americans die each flu season, according to the CDC.
Flu usually peaks in midwinter. Symptoms can include fever, cough, runny nose, head and body aches and fatigue. Some people also suffer vomiting and diarrhea, and some develop pneumonia or other severe complications.
A strain of swine flu that hit in 2009 caused a wave of cases in the spring and then again in the early fall. But that was considered a unique type of flu, distinct from the conventional strains that circulate every year.