CDC also added that 4.6 percent of all the flu viruses analyzed so far showed signs of resistance to oseltamivir (Tamiflu), the most commonly used antiviral flu drug. That represents a "real increase" in resistance but is not yet a major worry.
Overall, 8.1 percent of the influenza type A viruses tested by the CDC are resistant to Tamiflu. In past years, less than 1 percent of the viruses have been resistant to the drug.
This year's flu vaccine is not suited for some of the strains currently circulating in the United States. Some strains are also becoming resistant to a common antiviral medication.
Dr. Joe Bresee, chief of the epidemiology and prevention branch in the CDC's Influenza Division, "After relatively low levels of influenza activity in the early part of the season, since January, influenza activity has been picking up in the nation."
"This season, we are seeing more disease out there and higher rates of hospitalizations and deaths than we've seen in the last couple of years," Bresee added.
The virus strain most common in the U.S. is the influenza A H3N2 strain, and it's one strain not included in this year's vaccine. Also, this year's vaccine is not well matched against influenza type B.
The World Health Organization on Thursday included the H3N2 strain and other strains not in this year's vaccine. The widespread flu activity this week is up from 31 last week. And, as of Feb. 9, 10 children, in age from 4 months to 14 years, have died from influenza this year.
According to CDC, an estimated 5 percent to 20 percent of the U.S. population suffers from the flu each year. More than 200,000 people are hospitalized from flu complications, and about 36,000 people die from the disease.
