A deadly influenza strain has the U.S. in the grip of what could develop into a “severe” flu season, with widespread cases already reported in 36 states, a Centers for Disease Control and Prevention expert said Tuesday.
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The H3N2 strain, the most common flu virus this season, began mutating shortly after U.S. health experts created this year’s vaccine — rendering it less effective compared to past flu vaccines, Dr. Michael Jhung, a medical officer with the CDC’s influenza division, told FoxNews.com.

The H3N2 virus was predominant during the 2012-13, 2007-08 and 2003-04 flu seasons, the CDC said earlier this month. Because it’s been associated with particularly harsh flu seasons in the past, the CDC believes the 2014-15 season could be a “severe” one, Jhung said.

But he cautioned that this year’s flu season — which began in late November and is expected to continue through April — won’t be “terribly severe.”

“We’re seeing things that we see every year,” Jhung said. “We’re not seeing dramatically higher levels of flu activity than we see every year.”

The CDC says that those at high risk from influenza include children younger than 5 years (especially those younger than 2 years); adults 65 years and older; pregnant women; and people with certain chronic health conditions such as asthma, diabetes, heart or lung disease, and kidney disease.