Influenza was reported as widespread by 36 states last week, with some states reporting deaths from the virus, according to the Centers for Disease Control and Prevention.
Twenty-one of the 36 states experienced high levels of activity in the week ending Dec. 23, according to the CDC report released Friday. They are Alabama, Arizona, Arkansas, California, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Nebraska, Nevada, New Mexico, Oklahoma, Oregon, South Carolina, Tennessee, Texas, and West Virginia.
Almost a dozen total deaths have been reported in California, North Carolina and South Carolina.
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Data: Centers for Disease Control and Prevention
Experts had said that the flu seaon may be more severe this year, with the dominant strain being H3N2.
"Typically in years when the predominant strain is H3N2, there are more hospitalizations, more severe disease and people tend to get sicker," Dr. Michael Ison, a professor of infectious disease and organ transplantation at Northwestern University’s Feinberg School of Medicine, told NBC News.
And the vaccine available in the U.S. was reported as only 10 percent effective in preventing illness from H3N2. However, while those vaccinated can still get sick, they typically experience a milder form of the illness.
The CDC began bracing for the season in September, campaigning for flu vaccinations and advising the public about influenza symptoms, which can appear suddenly. Symptoms can include fever, sore throat, cough, runny nose, chills and body aches, headaches, vomiting and diarrhea.
The flu virus has had millions of affected cases in the U.S. each year since 2010, according to the CDC. Those cases resulted in between 12,000 and 56,000 deaths each year.