DENVER -- If you think there have been a lot of people out sick at work, you're not imagining things.

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The number of confirmed flu cases in Colorado continues to rise dramatically -- up more than 300 percent over last week. The state already has reported 693 cases, months before the flu season is expected to peak.

Doctors say it's because this year's flu vaccine does not exactly match the strain that's making people sick.

Officials with the Centers For Disease Control and Prevention say the mismatch is serious and this flu season could be worse than usual. However, health experts are still recommending that people get flu shots because the vaccine can help some from getting sick and avoid a potentially staggering flu season nationwide.

"Each year in the United States, 36,000 people die from complications related to the flu," Health and Human Services Secretary Tommy Thompson said. "There is plenty of vaccine to meet demand, so protect yourself and those who you love and go out and get vaccinated right now."

The strongest flu outbreaks so far have been in Texas and Colorado, the CDC said.

At the University of Colorado in Boulder, the health clinic had diagnosed 20 students a day with the flu. At a middle school in Fort Collins, more than 20 percent of the student population is out sick with the flu.

Last year, Colorado has 2,681 flu cases, including 19 fatalities. Another 748 died from flu-related complications.

The strain of flu showing up this year is part of a deadly group called H3N2, a type of flu that leads to more deaths and hospitalizations than other flu strains, health officials said.

But because this year's flu vaccine targets a slightly different type of H3N2 flu than patients are getting, doctors have no idea how well the vaccine will work. The virus changes slightly over time, a change doctors call "drift," which is why doctors suggest getting a new flu shot every year.

The shot is recommended for adults over 50, children between 6 months and 2 years, people with chronic medical conditions and people who work in health care

To find out where there's a flu shot clinic near you call (888) 692-0269.