Reporting by Kari Rea

Flu season has hit the U.S. particularly hard this year and the widespread outbreak has officially been declared an epidemic by the Centers for Disease Control and Prevention. High flu activity is reported in 22 states, with increased hospitalizations across the country.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said today that Americans are in for a rough flu season.

"If you look at the trajectory, it's not going to be a good year. It's going to be a bad year," Fauci told ABC's Martha Raddatz on "This Week." "How bad it's going to be will depend on how it actually evolves."

National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci on 'This Week'

Children, people over age 65, and those with weak immune systems are especially vulnerable to the flu. So far this year, 15 children have died from flu complications, while dozens more flu deaths have been reported nationwide.

Fauci told ABC News that the spread of the virus is "difficult to predict," but that experts track "patterns" of influenza cases across the country.

"The one thing about the flu that you can be sure, it's really unpredictable," Fauci said when asked how widely the flu could spread. "At the end of the day, it just devolves and it's difficult to predict."

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Fauci said this year's vaccine is only 33 percent effective in preventing the flu because the virus started to "drift" and mutate after the vaccine was already manufactured.

Though this year's vaccine is not a strong match for the most prevalent strain of the flu, Fauci still encouraged people to get vaccinated.

"Even though it isn't a good match to what's circulating… getting vaccinated can give you cross protection. It could be the difference between getting very sick or just being mildly sick, the difference between being hospitalized or not," Fauci said. "So we strongly recommend people getting vaccinated."

Despite the 67 percent ineffectiveness rate of this year's vaccine, Fauci recommended that those at high risk - such as children and the elderly - seek treatment if they feel ill.

"Particularly people at high risk … should get an anti-viral drug. They should see their physicians, because the anti-influenza drugs can be very, very helpful for people, particularly at high risk," Fauci said.

Unfortunately, there's no light at the end of the tunnel for those hoping to avoid the flu this year: Flu season peaks between December and February and can last as late as May.

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