Utah's still having a relatively mild flu season compared to most of the nation, but activity is increasing across all the states.

And it's complicated by the fact that about 60 percent of the confirmed flu cases are subtypes that do not match the flu vaccine given to Americans this year, officials at the Centers for Disease Control and Prevention said Friday.

Utah is one of five states reporting "regional" activity, which means it's slightly below threshold levels, according to Charla Haley, a Utah Department of Health spokeswoman. Forty-four states, including those surrounding Utah, now report widespread activity.

But influenzalike illnesses here have increased 50 percent in the past two weeks, and hospitalizations have doubled — most of them involving patients in high-risk groups. Of those, she said, 40 percent are children 4 and under. Utah has not seen any influenza-related children's deaths, while 10 children have died nationwide. Some of those children also had staphylococcus aureus, an example of co-existing illness that has health officials concerned.

This flu 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," CDC chief flu epidemiologist Dr. Joe Bresee said during a media teleconference Friday. It's "not atypical if you look at the last 20 years."

Haley said 70 percent of the state's hospitalizations were due to influenza A virus, 19 percent to influenza B and 11 percent are of unknown type. No subtypes were given.

Prior to each flu season, health experts determine the makeup of the upcoming season's flu vaccine, and most of the time their best guess is pretty good. In 16 of the past 19 years, they've accurately predicted the strains and subtypes that will circulate most widely.

This time, though, they missed on the A strain subtype (H3N2) and the B strain. The majority of the A strain (H1N1) viruses circulating are a good match. But the CDC and local health officials are encouraging people to get the vaccine if they haven't already because it provides at least partial protection against related strains and reduces the risk for flu-related complications and deaths. Good hygiene and common sense — stay home if you're sick — are key to preventing spread.

Thursday, the World Health Organization recommended that next year's vaccine be entirely different than the current one and said it should include H3N2, which has traditionally been associated with more complications and deaths. A decision by U.S. officials will be made next week.

There's another issue, according to Bresee: A growing number of influenza A (H1N1) viruses circulating in the United States are resistant to the popular Tamiflu antiviral. Last year, the number was fewer than 1 percent. No resistance has been found to Relenza so far.

The CDC is not suggesting changes to current recommendations on use of influenza antivirals, Bresee said.

Utah Department of Health posts a statewide summary of lab-confirmed influenza cases online each Wednesday afternoon at www.health.utah.gov/epi. The current one, which tracks activity through Feb. 9, says 97 people have been hospitalized with flu.

E-mail: lois@desnews.com