TUESDAY, Dec. 4 (HealthDay News) -- Flu season descended on the United States early and hard this year, with significant increases in flu activity observed in just the past two weeks, according to a Nov. 30 weekly surveillance report published by the U.S. Centers for Disease Control and Prevention.

By late November, 48 states plus Puerto Rico had reported laboratory confirmed flu cases, and influenza-like illness (ILI) activity was higher in some parts of the country than it was at any point during the last flu season. ILI activity is most predominant in the southeast and south-central portions of the country, but levels are growing nationally.

A partial explanation for the severity of this year's flu season is that the predominant strain, influenza A H3N2, is particularly virulent. Incidentally, the increased activity in ILI this year overlaps with National Influenza Vaccination Week, Dec. 2 through 8.

"Increasing flu activity should be a wake-up call. For anyone who has put off vaccination: It's time to get your flu vaccine now," Melinda Wharton, M.D., acting director of the CDC's National Center for Immunization and Respiratory Diseases, said in a statement.

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