FRIDAY, Feb. 15 (HealthDay News) -- Widespread flu activity now exists in virtually every state, and many of the infections are being caused by some strains not covered by this year's influenza vaccine, U.S. health officials said Friday.

"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," Dr. Joe Bresee, chief of the branch of epidemiology and prevention at the U.S. Centers for Disease Control and Prevention's Influenza Division, said during a teleconference.

"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.

Much of the increased activity owes to the fact that this year's flu vaccine isn't a match for some of the strains currently circulating in the United States, and some strains are becoming resistant to a common antiviral medication. The CDC reported last week that this year's flu vaccine doesn't match two of the three strains of influenza circulating in the United States.

"Slightly more than half of the viruses that we are looking at in our lab are viruses that are different than the vaccine strain," Bresee said. "So, they may not be well covered by the vaccine."

The virus strain most common in the United States right now 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, Bresee said.

The World Health Organization announced Thursday its recommendation for next year's flu vaccine, and it includes vaccine against the H3N2 strain and other strains not in this year's vaccine, Bresee said.

Complicating matters, some of this year's influenza type A virus is showing resistance to the antiviral drug Tamiflu. 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, Bresee said.

"This represents a real increase in resistance," he said.

Forty-four states reported widespread flu activity this week, up from 31 last week. And, as of Feb. 9, 10 children have died from influenza this year.

"This is not particularly unexpected," Bresee said. "We may see more pediatric deaths before the season is finished."

The children ranged in age from 4 months to 14 years. During the last three years, flu-related deaths among children have ranged from 46 to 74, Bresee said.

Even though this year's vaccine isn't a good match for most of the circulating flu virus, the CDC continues to recommend that people get inoculated. The reason: The vaccine still offers partial protection and can reduce the risk of flu-related complications.

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. Some people, such as older individuals, young children, and people with certain health conditions (such as asthma, diabetes or heart disease), are at high risk for serious flu complications, according to the CDC.

More information

For more on flu, visit the CDC.