BOSTON (CBS/AP) –Flu season appears to be in full swing in Massachusetts, with health officials reporting 860 cases statewide.

Health care providers are seeing an uptick in the H1N1 strain that was prevalent in 2009. H1N1 or “Swine Flu” typically causes more illness in children and young adults, compared to older adults, although severe illness is possible in all age groups.

READ MORE: Body Of Missing Five-Year-Old Elijah Lewis Believed To Be Found In Abington

The CDC says this year’s vaccine does protect against the H1N1 strain.

The Center for Disease Control says hardest hit areas include Massachusetts, New York and Pennsylvania.

Flu is usually the worst in January or February, although last year the illness hit the U.S. very hard in early December.

READ MORE: Richard Seymour's Full Patriots Hall Of Fame Induction Speech

According to the Centers for Disease Control and Prevention, last year’s flu season landed about 381,000 people in the hospital nationwide. That’s the most since at least 2005.

A contributing factor: The flu vaccine was only 51 percent effective overall.

The CDC released an alert on the H1N1 strain last week.

MORE NEWS: COVID Outbreak Reported In New Hampshire's Largest Jail

“While it is not possible to predict which influenza viruses will predominate during the entire 2013-14 influenza season, pH1N1 has been the predominant circulating virus so far. For the 2013-14 season, if pH1N1 virus continues to circulate widely, illness that disproportionately affects young and middle-aged adults may occur,” the alert said.

(TM and © Copyright 2013 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2013 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)