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.

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.

So far, the CDC has recorded 15 flu-related deaths in children across the U.S. this season, though Jhung said it's too soon to make comparisons to past years. Overall, there were 109 flu-related deaths in kids last year, and 171 in the 2012-13 season. There were more hospitalizations so far this year compared to the same time in the previous two seasons, however.

The CDC does not compile similar data for flu-related deaths in adults.

Jhung said that because this year's vaccine might not be as effective as that of past years, people at high risk from the flu, including the elderly, young children and people with underlying chronic medical conditions should obtain flu anti-virals.

And he said Americans who haven't gotten the vaccine yet should get it, adding that only 40 percent of people who the CDC believes should get vaccinated have done so thus far.

"We're not even halfway through the flu season," he said. "It's certainly not too late to get vaccinated."